diff --git a/pint/quantity.py b/pint/quantity.py
index 7d20199..76e8f4f 100644
--- a/pint/quantity.py
+++ b/pint/quantity.py
@@ -174,13 +174,13 @@ class _Quantity(SharedRegistryObject):
     def unitless(self):
         """Return true if the quantity does not have units.
         """
-        return not bool(self.to_base_units()._units)
+        return not bool(self.to_root_units()._units)
 
     @property
     def dimensionless(self):
         """Return true if the quantity is dimensionless.
         """
-        tmp = self.to_base_units()
+        tmp = self.to_root_units()
 
         return not bool(tmp.dimensionality)
 
@@ -243,6 +243,26 @@ class _Quantity(SharedRegistryObject):
 
         return self.__class__(magnitude, other)
 
+    def ito_root_units(self):
+        """Return Quantity rescaled to base units
+        """
+
+        _, other = self._REGISTRY._get_root_units(self._units)
+
+        self._magnitude = self._convert_magnitude(other)
+        self._units = other
+
+        return None
+
+    def to_root_units(self):
+        """Return Quantity rescaled to base units
+        """
+        _, other = self._REGISTRY._get_root_units(self._units)
+
+        magnitude = self._convert_magnitude_not_inplace(other)
+
+        return self.__class__(magnitude, other)
+
     def ito_base_units(self):
         """Return Quantity rescaled to base units
         """
@@ -263,6 +283,7 @@ class _Quantity(SharedRegistryObject):
 
         return self.__class__(magnitude, other)
 
+
     def to_compact(self, unit=None):
         """Return Quantity rescaled to compact, human-readable units.
 
@@ -603,14 +624,14 @@ class _Quantity(SharedRegistryObject):
         if not self._ok_for_muldiv(no_offset_units_self):
             raise OffsetUnitCalculusError(self._units, other._units)
         elif no_offset_units_self == 1 and len(self._units) == 1:
-                self.ito_base_units()
+                self.ito_root_units()
 
         no_offset_units_other = len(other._get_non_multiplicative_units())
 
         if not other._ok_for_muldiv(no_offset_units_other):
             raise OffsetUnitCalculusError(self._units, other._units)
         elif no_offset_units_other == 1 and len(other._units) == 1:
-            other.ito_base_units()
+            other.ito_root_units()
 
         self._magnitude = magnitude_op(self._magnitude, other._magnitude)
         self._units = units_op(self._units, other._units)
@@ -665,14 +686,14 @@ class _Quantity(SharedRegistryObject):
         if not self._ok_for_muldiv(no_offset_units_self):
             raise OffsetUnitCalculusError(self._units, other._units)
         elif no_offset_units_self == 1 and len(self._units) == 1:
-            new_self = self.to_base_units()
+            new_self = self.to_root_units()
 
         no_offset_units_other = len(other._get_non_multiplicative_units())
 
         if not other._ok_for_muldiv(no_offset_units_other):
             raise OffsetUnitCalculusError(self._units, other._units)
         elif no_offset_units_other == 1 and len(other._units) == 1:
-            other = other.to_base_units()
+            other = other.to_root_units()
 
         magnitude = magnitude_op(new_self._magnitude, other._magnitude)
         units = units_op(new_self._units, other._units)
@@ -718,7 +739,7 @@ class _Quantity(SharedRegistryObject):
         if not self._ok_for_muldiv(no_offset_units_self):
             raise OffsetUnitCalculusError(self._units, '')
         elif no_offset_units_self == 1 and len(self._units) == 1:
-            self = self.to_base_units()
+            self = self.to_root_units()
 
         return self.__class__(other_magnitude / self._magnitude, 1 / self._units)
 
@@ -732,7 +753,7 @@ class _Quantity(SharedRegistryObject):
         if not self._ok_for_muldiv(no_offset_units_self):
             raise OffsetUnitCalculusError(self._units, '')
         elif no_offset_units_self == 1 and len(self._units) == 1:
-            self = self.to_base_units()
+            self = self.to_root_units()
 
         return self.__class__(other_magnitude // self._magnitude, 1 / self._units)
 
@@ -803,12 +824,12 @@ class _Quantity(SharedRegistryObject):
             else:
                 if not self._is_multiplicative:
                     if self._REGISTRY.autoconvert_offset_to_baseunit:
-                        new_self = self.to_base_units()
+                        new_self = self.to_root_units()
                     else:
                         raise OffsetUnitCalculusError(self._units)
 
                 if getattr(other, 'dimensionless', False):
-                    units = new_self._units ** other.to_base_units().magnitude
+                    units = new_self._units ** other.to_root_units().magnitude
                 elif not getattr(other, 'dimensionless', True):
                     raise DimensionalityError(self._units, 'dimensionless')
                 else:
@@ -828,7 +849,7 @@ class _Quantity(SharedRegistryObject):
             if isinstance(self._magnitude, ndarray):
                 if np.size(self._magnitude) > 1:
                     raise DimensionalityError(self._units, 'dimensionless')
-            new_self = self.to_base_units()
+            new_self = self.to_root_units()
             return other**new_self._magnitude
 
     def __abs__(self):
@@ -880,8 +901,8 @@ class _Quantity(SharedRegistryObject):
         if self.dimensionality != other.dimensionality:
             raise DimensionalityError(self._units, other._units,
                                       self.dimensionality, other.dimensionality)
-        return op(self.to_base_units().magnitude,
-                  other.to_base_units().magnitude)
+        return op(self.to_root_units().magnitude,
+                  other.to_root_units().magnitude)
 
     __lt__ = lambda self, other: self.compare(other, op=operator.lt)
     __le__ = lambda self, other: self.compare(other, op=operator.le)
@@ -1089,9 +1110,9 @@ class _Quantity(SharedRegistryObject):
 
         try:
             if isinstance(value, self.__class__):
-                factor = self.__class__(value.magnitude, value._units / self._units).to_base_units()
+                factor = self.__class__(value.magnitude, value._units / self._units).to_root_units()
             else:
-                factor = self.__class__(value, self._units ** (-1)).to_base_units()
+                factor = self.__class__(value, self._units ** (-1)).to_root_units()
 
             if isinstance(factor, self.__class__):
                 if not factor.dimensionless:
@@ -1170,7 +1191,7 @@ class _Quantity(SharedRegistryObject):
                             if unt == 'radian':
                                 mobjs.append(getattr(other, 'magnitude', other))
                             else:
-                                factor, units = self._REGISTRY._get_base_units(unt)
+                                factor, units = self._REGISTRY._get_root_units(unt)
                                 if units and units != UnitsContainer({'radian': 1}):
                                     raise DimensionalityError(units, dst_units)
                                 mobjs.append(getattr(other, 'magnitude', other) * factor)
diff --git a/pint/systems.py b/pint/systems.py
index bea5f97..6b55cf8 100644
--- a/pint/systems.py
+++ b/pint/systems.py
@@ -370,108 +370,48 @@ class System(object):
         return system
 
 
-#: These dictionaries will be part of the registry
-#: :type: dict[str, Group | System]
-_groups_systems = dict()
-_root_group = Group('root', _groups_systems)
+class GSManager(object):
 
+    def __init__(self):
+        #: :type: dict[str, Group | System]
+        self._groups_systems = dict()
+        self._root_group = Group('root', self._groups_systems)
 
-# These methods will be included in the registry, upgrading the existing ones.
+    def get_group(self, name, create_if_needed=True):
+        """Return a Group.
 
-def get_group(registry, name, create_if_needed=True):
-    """Return a Group.
+        :param name: Name of the group.
+        :param create_if_needed: Create a group if not Found. If False, raise an Exception.
+        :return: Group
+        """
+        try:
+            return self._groups_systems[name]
+        except KeyError:
+            if create_if_needed:
+                if name == 'root':
+                    raise ValueError('The name root is reserved.')
+                return Group(name, self._groups_systems)
+            else:
+                raise KeyError('No group %s found.' % name)
 
-    :param registry:
-    :param name: Name of the group to be
-    :param create_if_needed: Create a group if not Found. If False, raise an Exception.
-    :return: Group
-    """
-    if name == 'root':
-        raise ValueError('The name root is reserved.')
+    def get_system(self, name, create_if_needed=True):
+        """Return a Group.
 
-    try:
-        return _groups_systems[name]
-    except KeyError:
-        if create_if_needed:
-            return Group(name, _groups_systems)
-        else:
-            raise KeyError('No group %s found.' % name)
+        :param name: Name of the system
+        :param create_if_needed: Create a group if not Found. If False, raise an Exception.
+        :return: System
+        """
 
+        try:
+            return self._groups_systems[name]
+        except KeyError:
+            if create_if_needed:
+                return System(name, self._groups_systems)
+            else:
+                raise KeyError('No system found named %s.' % name)
 
-def get_system(registry, name, create_if_needed=True):
-    """Return a Group.
+    def __getitem__(self, item):
+        if item in self._groups_systems:
+            return self._groups_systems[item]
 
-    :param registry:
-    :param name: Name of the group to be
-    :param create_if_needed: Create a group if not Found. If False, raise an Exception.
-    :return: System
-    """
-
-    try:
-        return _groups_systems[name]
-    except KeyError:
-        if create_if_needed:
-            return System(name, _groups_systems)
-        else:
-            raise KeyError('No system %s found.' % name)
-
-
-def get_compatible_units(registry, input_units, group_or_system=None):
-    """
-    :param registry:
-    :param input_units:
-    :param group_or_system:
-    :type group_or_system: Group | System
-    :return:
-    """
-    ret = registry.get_compatible_units(input_units)
-
-    if not group_or_system:
-        return ret
-
-    members = _groups_systems[group_or_system].members
-
-    # This will not be necessary after integration with the registry as it has a strings intermediate
-    members = frozenset((getattr(registry, member) for member in members))
-
-    return ret.intersection(members)
-
-
-# Current get_base_units will be renamed to get_root_units
-#
-# Not sure yet how to deal with the cache.
-# Should we cache get_root_units, get_base_units or both?
-# - get_base_units will need to be invalidated when the system is changed (How often will this happen?)
-# - get_root_units will not need to be invalidated.
-
-def get_base_units(registry, input_units, check_nonmult=True, system=None):
-    """
-    :param registry:
-    :param input_units:
-    :param check_nonmult:
-    :param system: System
-    :return:
-    """
-    factor, units = registry.get_base_units(input_units, check_nonmult)
-
-    if not system:
-        return factor, units
-
-    # This will not be necessary after integration with the registry as it has a UnitsContainer intermediate
-    units = to_units_container(units, registry)
-
-    destination_units = UnitsContainer()
-
-    bu = _groups_systems[system].base_units
-
-    for unit, value in units.items():
-        if unit in bu:
-            new_unit = bu[unit]
-            new_unit = to_units_container(new_unit, registry)
-            destination_units *= new_unit ** value
-        else:
-            destination_units *= UnitsContainer({unit: value})
-
-    base_factor = registry.convert(factor, units, destination_units)
-
-    return base_factor, destination_units
+        raise KeyError('No group or system found named %s.' % item)
diff --git a/pint/testsuite/test_systems.py b/pint/testsuite/test_systems.py
index 069cbc7..ecc71ba 100644
--- a/pint/testsuite/test_systems.py
+++ b/pint/testsuite/test_systems.py
@@ -4,7 +4,7 @@ from __future__ import division, unicode_literals, print_function, absolute_impo
 
 
 from pint import UnitRegistry
-from pint.systems import Group, get_compatible_units, get_group, System, get_base_units
+from pint.systems import Group, System
 
 from pint.testsuite import QuantityTestCase
 
@@ -187,9 +187,9 @@ class TestGroup(QuantityTestCase):
     def test_get_compatible_units(self):
         ureg = UnitRegistry()
 
-        g = get_group(ureg, 'imperial')
+        g = ureg.get_group('imperial')
         g.add_units('inch', 'yard', 'pint')
-        c = get_compatible_units(ureg, 'meter', 'imperial')
+        c = ureg.get_compatible_units('meter', 'imperial')
         self.assertEqual(c, frozenset([ureg.inch, ureg.yard]))
 
 
@@ -238,10 +238,10 @@ class TestSystem(QuantityTestCase):
         sysname = 'mysys1'
         ureg = UnitRegistry()
 
-        g = get_group(ureg, 'imperial')
+        g = ureg.get_group('imperial')
 
         g.add_units('inch', 'yard', 'pint')
-        c = get_compatible_units(ureg, 'meter', 'imperial')
+        c = ureg.get_compatible_units('meter', 'imperial')
         self.assertEqual(c, frozenset([ureg.inch, ureg.yard]))
 
         lines = ['@system %s using imperial' % sysname,
@@ -249,7 +249,7 @@ class TestSystem(QuantityTestCase):
                  ]
 
         s = System.from_lines(lines, lambda x: x, g._groups_systems)
-        c = get_compatible_units(ureg, 'meter', sysname)
+        c = ureg.get_compatible_units('meter', sysname)
         self.assertEqual(c, frozenset([ureg.inch, ureg.yard]))
 
     def test_get_base_units(self):
@@ -257,7 +257,7 @@ class TestSystem(QuantityTestCase):
 
         ureg = UnitRegistry()
 
-        g = get_group(ureg, 'imperial')
+        g = ureg.get_group('imperial')
         g.add_units('inch', 'yard', 'pint')
 
         lines = ['@system %s using imperial' % sysname,
@@ -267,11 +267,11 @@ class TestSystem(QuantityTestCase):
         s = System.from_lines(lines, ureg.get_base_units, g._groups_systems)
 
         # base_factor, destination_units
-        c = get_base_units(ureg, 'inch', system=sysname)
+        c = ureg.get_base_units('inch', system=sysname)
         self.assertAlmostEqual(c[0], 1)
         self.assertEqual(c[1], {'inch': 1})
 
-        c = get_base_units(ureg, 'cm', system=sysname)
+        c = ureg.get_base_units('cm', system=sysname)
         self.assertAlmostEqual(c[0], 1./2.54)
         self.assertEqual(c[1], {'inch': 1})
 
@@ -280,9 +280,9 @@ class TestSystem(QuantityTestCase):
 
         ureg = UnitRegistry()
 
-        g = get_group(ureg, 'imperial')
+        g = ureg.get_group('imperial')
         g.add_units('inch', 'yard', 'pint')
-        c = get_compatible_units(ureg, 'meter', 'imperial')
+        c = ureg.get_compatible_units('meter', 'imperial')
 
         lines = ['@system %s using imperial' % sysname,
                  'pint:meter',
@@ -291,19 +291,19 @@ class TestSystem(QuantityTestCase):
         s = System.from_lines(lines, ureg.get_base_units, g._groups_systems)
 
         # base_factor, destination_units
-        c = get_base_units(ureg, 'inch', system=sysname)
+        c = ureg.get_base_units('inch', system=sysname)
         self.assertAlmostEqual(c[0], 0.326, places=3)
         self.assertEqual(c[1], {'pint': 1./3})
 
-        c = get_base_units(ureg, 'cm', system=sysname)
+        c = ureg.get_base_units('cm', system=sysname)
         self.assertAlmostEqual(c[0], 0.1283, places=3)
         self.assertEqual(c[1], {'pint': 1./3})
 
-        c = get_base_units(ureg, 'inch**2', system=sysname)
+        c = ureg.get_base_units('inch**2', system=sysname)
         self.assertAlmostEqual(c[0], 0.326**2, places=3)
         self.assertEqual(c[1], {'pint': 2./3})
 
-        c = get_base_units(ureg, 'cm**2', system=sysname)
+        c = ureg.get_base_units('cm**2', system=sysname)
         self.assertAlmostEqual(c[0], 0.1283**2, places=3)
         self.assertEqual(c[1], {'pint': 2./3})
 
@@ -312,7 +312,7 @@ class TestSystem(QuantityTestCase):
 
         ureg = UnitRegistry()
 
-        g = get_group(ureg, 'imperial')
+        g = ureg.get_group('imperial')
         g.add_units('inch', 'yard', 'pint')
 
         lines = ['@system %s using imperial' % sysname,
@@ -322,10 +322,10 @@ class TestSystem(QuantityTestCase):
         s = System.from_lines(lines, ureg.get_base_units, g._groups_systems)
 
         # base_factor, destination_units
-        c = get_base_units(ureg, 'inch', system=sysname)
+        c = ureg.get_base_units('inch', system=sysname)
         self.assertAlmostEqual(c[0], 0.0568, places=3)
         self.assertEqual(c[1], {'mph': 1, 'second': 1})
 
-        c = get_base_units(ureg, 'kph', system=sysname)
+        c = ureg.get_base_units('kph', system=sysname)
         self.assertAlmostEqual(c[0], .6214, places=4)
         self.assertEqual(c[1], {'mph': 1})
diff --git a/pint/unit.py b/pint/unit.py
index 7ca1591..356e693 100644
--- a/pint/unit.py
+++ b/pint/unit.py
@@ -37,6 +37,7 @@ from .errors import (DimensionalityError, UndefinedUnitError,
                      DefinitionSyntaxError, RedefinitionError)
 
 from .pint_eval import build_eval_tree
+from . import systems
 
 class _Unit(SharedRegistryObject):
     """Implements a class to describe a unit supporting math operations.
@@ -265,6 +266,9 @@ class UnitRegistry(object):
         #: Map dimension name (string) to its definition (DimensionDefinition).
         self._dimensions = {}
 
+        #: :type: systems.GSManager
+        self._gsmanager = systems.GSManager()
+
         #: Map unit name (string) to its definition (UnitDefinition).
         #: Might contain prefixed units.
         self._units = {}
@@ -289,6 +293,9 @@ class UnitRegistry(object):
         #: Maps dimensionality (UnitsContainer) to Units (str)
         self._dimensional_equivalents = dict()
 
+        #: Maps dimensionality (UnitsContainer) to Dimensionality (UnitsContainer)
+        self._root_units_cache = dict()
+
         #: Maps dimensionality (UnitsContainer) to Dimensionality (UnitsContainer)
         self._base_units_cache = dict()
 
@@ -344,6 +351,25 @@ class UnitRegistry(object):
         self.Unit.default_format = value
         self.Quantity.default_format = value
 
+    def get_group(self, name, create_if_needed=True):
+        """Return a Group.
+
+        :param name: Name of the group to be
+        :param create_if_needed: Create a group if not Found. If False, raise an Exception.
+        :return: Group
+        """
+        return self._gsmanager.get_group(name, create_if_needed)
+
+    def get_system(self, name, create_if_needed=True):
+        """Return a Group.
+
+        :param registry:
+        :param name: Name of the group to be
+        :param create_if_needed: Create a group if not Found. If False, raise an Exception.
+        :return: System
+        """
+        return self._gsmanager.get_system(name, create_if_needed)
+
     def add_context(self, context):
         """Add a context object to the registry.
 
@@ -611,10 +637,10 @@ class UnitRegistry(object):
                 try:
                     uc = ParserHelper.from_word(unit_name)
 
-                    bu = self._get_base_units(uc)
+                    bu = self._get_root_units(uc)
                     di = self._get_dimensionality(uc)
 
-                    self._base_units_cache[uc] = bu
+                    self._root_units_cache[uc] = bu
                     self._dimensionality_cache[uc] = di
 
                     if not prefixed:
@@ -726,8 +752,8 @@ class UnitRegistry(object):
                 if reg.reference is not None:
                     self._get_dimensionality_recurse(reg.reference, exp2, accumulator)
 
-    def get_base_units(self, input_units, check_nonmult=True):
-        """Convert unit or dict of units to the base units.
+    def get_root_units(self, input_units, check_nonmult=True):
+        """Convert unit or dict of units to the root units.
 
         If any unit is non multiplicative and check_converter is True,
         then None is returned as the multiplicative factor.
@@ -741,12 +767,12 @@ class UnitRegistry(object):
         """
         input_units = to_units_container(input_units)
 
-        f, units = self._get_base_units(input_units, check_nonmult=True)
+        f, units = self._get_root_units(input_units, check_nonmult)
 
         return f, self.Unit(units)
 
-    def _get_base_units(self, input_units, check_nonmult=True):
-        """Convert unit or dict of units to the base units.
+    def _get_root_units(self, input_units, check_nonmult=True):
+        """Convert unit or dict of units to the root units.
 
         If any unit is non multiplicative and check_converter is True,
         then None is returned as the multiplicative factor.
@@ -762,11 +788,11 @@ class UnitRegistry(object):
             return 1., UnitsContainer()
 
         # The cache is only done for check_nonmult=True
-        if check_nonmult and input_units in self._base_units_cache:
-            return self._base_units_cache[input_units]
+        if check_nonmult and input_units in self._root_units_cache:
+            return self._root_units_cache[input_units]
 
         accumulators = [1., defaultdict(float)]
-        self._get_base_units_recurse(input_units, 1.0, accumulators)
+        self._get_root_units_recurse(input_units, 1.0, accumulators)
 
         factor = accumulators[0]
         units = UnitsContainer(dict((k, v) for k, v in accumulators[1].items()
@@ -780,7 +806,63 @@ class UnitRegistry(object):
 
         return factor, units
 
-    def _get_base_units_recurse(self, ref, exp, accumulators):
+    def get_base_units(self, input_units, check_nonmult=True, system=None):
+        """Convert unit or dict of units to the base units.
+
+        If any unit is non multiplicative and check_converter is True,
+        then None is returned as the multiplicative factor.
+
+        :param input_units: units
+        :type input_units: UnitsContainer or str
+        :param check_nonmult: if True, None will be returned as the
+                              multiplicative factor if a non-multiplicative
+                              units is found in the final Units.
+        :return: multiplicative factor, base units
+        """
+        input_units = to_units_container(input_units)
+
+        f, units = self._get_base_units(input_units, check_nonmult, system)
+
+        return f, self.Unit(units)
+
+    def _get_base_units(self, input_units, check_nonmult=True, system=None):
+        """
+        :param registry:
+        :param input_units:
+        :param check_nonmult:
+        :param system: System
+        :return:
+        """
+
+        # The cache is only done for check_nonmult=True
+        if check_nonmult and input_units in self._base_units_cache:
+            return self._base_units_cache[input_units]
+
+        factor, units = self.get_root_units(input_units, check_nonmult)
+
+        if not system:
+            return factor, units
+
+        # This will not be necessary after integration with the registry as it has a UnitsContainer intermediate
+        units = to_units_container(units, self)
+
+        destination_units = UnitsContainer()
+
+        bu = self._gsmanager.get_system(system, False).base_units
+
+        for unit, value in units.items():
+            if unit in bu:
+                new_unit = bu[unit]
+                new_unit = to_units_container(new_unit, self)
+                destination_units *= new_unit ** value
+            else:
+                destination_units *= UnitsContainer({unit: value})
+
+        base_factor = self.convert(factor, units, destination_units)
+
+        return base_factor, destination_units
+
+    def _get_root_units_recurse(self, ref, exp, accumulators):
         for key in ref:
             exp2 = exp*ref[key]
             key = self.get_name(key)
@@ -790,19 +872,19 @@ class UnitRegistry(object):
             else:
                 accumulators[0] *= reg._converter.scale ** exp2
                 if reg.reference is not None:
-                    self._get_base_units_recurse(reg.reference, exp2,
+                    self._get_root_units_recurse(reg.reference, exp2,
                                                  accumulators)
 
-    def get_compatible_units(self, input_units):
+    def get_compatible_units(self, input_units, group_or_system=None):
         """
         """
         input_units = to_units_container(input_units)
 
-        equiv = self._get_compatible_units(input_units)
+        equiv = self._get_compatible_units(input_units, group_or_system)
 
         return frozenset(self.Unit(eq) for eq in equiv)
 
-    def _get_compatible_units(self, input_units):
+    def _get_compatible_units(self, input_units, group_or_system=None):
         """
         """
         if not input_units:
@@ -819,7 +901,11 @@ class UnitRegistry(object):
                 for node in nodes:
                     ret |= self._dimensional_equivalents[node]
 
-        return frozenset(ret)
+        if group_or_system:
+            members = self._gsmanager[group_or_system].members
+            return frozenset(ret.intersection(members))
+
+        return ret
 
     def convert(self, value, src, dst, inplace=False):
         """Convert value from some source to destination units.
@@ -927,7 +1013,7 @@ class UnitRegistry(object):
 
         # Here src and dst have only multiplicative units left. Thus we can
         # convert with a factor.
-        factor, units = self._get_base_units(src / dst)
+        factor, units = self._get_root_units(src / dst)
 
         # factor is type float and if our magnitude is type Decimal then
         # must first convert to Decimal before we can '*' the values