diff --git a/ament_package/__init__.py b/ament_package/__init__.py
index 329648d..f819bf5 100644
--- a/ament_package/__init__.py
+++ b/ament_package/__init__.py
@@ -94,7 +94,6 @@ def parse_package_string(data, *, filename=None):
 
     from .exceptions import InvalidPackage
     from .export import Export
-    from .group_dependency import GroupDependency
     from .package import Package
     from .person import Person
     from .url import Url
@@ -212,10 +211,9 @@ def parse_package_string(data, *, filename=None):
     pkg.conflicts = _get_dependencies(root, 'conflict')
     pkg.replaces = _get_dependencies(root, 'replace')
 
-    # group dependencies and membership
-    pkg.group_depends = [
-        GroupDependency(g) for g in _get_groups(root, 'group_depend')]
-    pkg.member_of_groups = _get_groups(root, 'member_of_group')
+    # group dependencies and memberships
+    pkg.group_depends = _get_group_dependencies(root, 'group_depend')
+    pkg.member_of_groups = _get_group_memberships(root, 'member_of_group')
 
     # exports
     export_node = _get_optional_node(root, 'export')
@@ -374,8 +372,23 @@ def _get_dependencies(parent, tagname):
     return depends
 
 
-def _get_groups(parent, tagname):
-    groups = []
+def _get_group_dependencies(parent, tagname):
+    from .group_dependency import GroupDependency
+    depends = []
+    for node in _get_nodes(parent, tagname):
+        depends.append(
+            GroupDependency(
+                _get_node_value(node),
+                condition=_get_node_attr(node, 'condition', default=None)))
+    return depends
+
+
+def _get_group_memberships(parent, tagname):
+    from .group_membership import GroupMembership
+    memberships = []
     for node in _get_nodes(parent, tagname):
-        groups.append(_get_node_value(node))
-    return groups
+        memberships.append(
+            GroupMembership(
+                _get_node_value(node),
+                condition=_get_node_attr(node, 'condition', default=None)))
+    return memberships
diff --git a/ament_package/condition.py b/ament_package/condition.py
new file mode 100644
index 0000000..2f3fbd2
--- /dev/null
+++ b/ament_package/condition.py
@@ -0,0 +1,76 @@
+# Copyright 2014 Open Source Robotics Foundation, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import pyparsing
+
+
+def evaluate_condition(condition, context):
+    if condition is None:
+        return True
+    expr = _get_condition_expression()
+    try:
+        parse_results = expr.parseString(condition, parseAll=True)
+    except pyparsing.ParseException as e:
+        raise ValueError(
+            "condition '%s' failed to parse: %s" % (condition, e))
+    return _evaluate(parse_results.asList()[0], context)
+
+
+_condition_expression = None
+
+
+def _get_condition_expression():
+    global _condition_expression
+    if not _condition_expression:
+        pp = pyparsing
+        operator = pp.Regex('==|!=').setName('operator')
+        identifier = pp.Word('$', pp.alphanums + '_', min=2)
+        value = pp.Word(pp.alphanums + '_-')
+        comparison_term = identifier | value
+        condition = pp.Group(comparison_term + operator + comparison_term)
+        _condition_expression = pp.operatorPrecedence(
+            condition, [
+                ('and', 2, pp.opAssoc.LEFT, ),
+                ('or', 2, pp.opAssoc.LEFT, ),
+            ])
+    return _condition_expression
+
+
+def _evaluate(parse_results, context):
+    if not isinstance(parse_results, list):
+        if parse_results.startswith('$'):
+            # get variable from context
+            return str(context.get(parse_results[1:], ''))
+        # return literal value
+        return parse_results
+
+    # recursion
+    assert len(parse_results) == 3
+
+    # handle logical operators
+    if parse_results[1] == 'and':
+        return _evaluate(parse_results[0], context) and \
+            _evaluate(parse_results[2], context)
+    if parse_results[1] == 'or':
+        return _evaluate(parse_results[0], context) or \
+            _evaluate(parse_results[2], context)
+
+    # handle comparison operators
+    assert parse_results[1] in ('==', '!=')
+    if parse_results[1] == '==':
+        return _evaluate(parse_results[0], context) == \
+            _evaluate(parse_results[2], context)
+    if parse_results[1] == '!=':
+        return _evaluate(parse_results[0], context) != \
+            _evaluate(parse_results[2], context)
diff --git a/ament_package/dependency.py b/ament_package/dependency.py
index 9f43463..884fa0b 100644
--- a/ament_package/dependency.py
+++ b/ament_package/dependency.py
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from ament_package.condition import evaluate_condition
+
 
 class Dependency:
     __slots__ = [
@@ -21,9 +23,12 @@ class Dependency:
         'version_eq',
         'version_gte',
         'version_gt',
+        'condition',
+        'evaluated_condition',
     ]
 
     def __init__(self, name, **kwargs):
+        self.evaluated_condition = None
         for attr in self.__slots__:
             value = kwargs[attr] if attr in kwargs else None
             setattr(self, attr, value)
@@ -41,3 +46,18 @@ def __eq__(self, other):
 
     def __str__(self):
         return self.name
+
+    def evaluate_condition(self, context):
+        """
+        Evaluate the condition.
+
+        The result is also stored in the member variable `evaluated_condition`.
+
+        :param context: A dictionary with key value pairs to replace variables
+          starting with $ in the condition.
+
+        :returns: True if the condition evaluates to True, else False
+        :raises: :exc:`ValueError` if the condition fails to parse
+        """
+        self.evaluated_condition = evaluate_condition(self.condition, context)
+        return self.evaluated_condition
diff --git a/ament_package/group_dependency.py b/ament_package/group_dependency.py
index 42edbcf..0e7838d 100644
--- a/ament_package/group_dependency.py
+++ b/ament_package/group_dependency.py
@@ -12,16 +12,22 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from ament_package.condition import evaluate_condition
+
 
 class GroupDependency:
     __slots__ = [
         'name',
+        'condition',
+        'evaluated_condition',
         'members',
     ]
 
-    def __init__(self, name, members=None):
+    def __init__(self, name, condition=None, members=None):
         self.name = name
+        self.condition = condition
         self.members = members
+        self.evaluated_condition = None
 
     def __eq__(self, other):
         if not isinstance(other, GroupDependency):
@@ -32,8 +38,23 @@ def __eq__(self, other):
     def __str__(self):
         return self.name
 
+    def evaluate_condition(self, context):
+        """
+        Evaluate the condition.
+
+        The result is also stored in the member variable `evaluated_condition`.
+
+        :param context: A dictionary with key value pairs to replace variables
+          starting with $ in the condition.
+
+        :returns: True if the condition evaluates to True, else False
+        :raises: :exc:`ValueError` if the condition fails to parse
+        """
+        self.evaluated_condition = evaluate_condition(self.condition, context)
+        return self.evaluated_condition
+
     def extract_group_members(self, packages):
         self.members = set()
         for pkg in packages:
-            if self.name in pkg.member_of_groups:
+            if self.name in [g.name for g in pkg.member_of_groups]:
                 self.members.add(pkg.name)
diff --git a/ament_package/group_membership.py b/ament_package/group_membership.py
new file mode 100644
index 0000000..6aec2ec
--- /dev/null
+++ b/ament_package/group_membership.py
@@ -0,0 +1,52 @@
+# Copyright 2017 Open Source Robotics Foundation, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from ament_package.condition import evaluate_condition
+
+
+class GroupMembership:
+    __slots__ = [
+        'name',
+        'condition',
+        'evaluated_condition',
+    ]
+
+    def __init__(self, name, condition=None):
+        self.name = name
+        self.condition = condition
+        self.evaluated_condition = None
+
+    def __eq__(self, other):
+        if not isinstance(other, GroupMembership):
+            return False
+        return all(getattr(self, attr) == getattr(other, attr)
+                   for attr in self.__slots__)
+
+    def __str__(self):
+        return self.name
+
+    def evaluate_condition(self, context):
+        """
+        Evaluate the condition.
+
+        The result is also stored in the member variable `evaluated_condition`.
+
+        :param context: A dictionary with key value pairs to replace variables
+          starting with $ in the condition.
+
+        :returns: True if the condition evaluates to True, else False
+        :raises: :exc:`ValueError` if the condition fails to parse
+        """
+        self.evaluated_condition = evaluate_condition(self.condition, context)
+        return self.evaluated_condition
diff --git a/ament_package/package.py b/ament_package/package.py
index 779cf2a..1c23f5e 100644
--- a/ament_package/package.py
+++ b/ament_package/package.py
@@ -101,6 +101,32 @@ def get_build_type(self):
             return build_type_exports[0]
         raise InvalidPackage('Only one <build_type> element is permitted.')
 
+    def evaluate_conditions(self, context):
+        """
+        Evaluate the conditions of all dependencies and memberships.
+
+        :param context: A dictionary with key value pairs to replace variables
+          starting with $ in the condition.
+
+        :raises: :exc:`ValueError` if any condition fails to parse
+        """
+        for attr in (
+            'build_depends',
+            'buildtool_depends',
+            'build_export_depends',
+            'buildtool_export_depends',
+            'exec_depends',
+            'test_depends',
+            'doc_depends',
+            'conflicts',
+            'replaces',
+            'group_depends',
+            'member_of_groups',
+        ):
+            conditionals = getattr(self, attr)
+            for conditional in conditionals:
+                conditional.evaluate_condition(context)
+
     def validate(self):
         """
         Ensure that all standards for packages are met.
@@ -179,7 +205,10 @@ def validate(self):
                         "The package must not '%s_depend' on a package with "
                         'the same name as this package' % dep_type)
 
-        if {d.name for d in self.group_depends} & set(self.member_of_groups):
+        if (
+            {d.name for d in self.group_depends} &
+            {g.name for g in self.member_of_groups}
+        ):
             errors.append(
                 "The package must not 'group_depend' on a package which it "
                 'also declares to be a member of')
diff --git a/test/test_package.py b/test/test_package.py
index 7371209..c0b6eb7 100644
--- a/test/test_package.py
+++ b/test/test_package.py
@@ -74,13 +74,18 @@ def test_init_dependency(self):
                          version_lte=2,
                          version_eq=3,
                          version_gte=4,
-                         version_gt=5)
+                         version_gt=5,
+                         condition='$foo == 23 and $bar != 42')
         self.assertEqual('foo', dep.name)
         self.assertEqual(1, dep.version_lt)
         self.assertEqual(2, dep.version_lte)
         self.assertEqual(3, dep.version_eq)
         self.assertEqual(4, dep.version_gte)
         self.assertEqual(5, dep.version_gt)
+        self.assertFalse(dep.evaluate_condition({'foo': 23, 'bar': 42}))
+        self.assertFalse(dep.evaluated_condition)
+        self.assertTrue(dep.evaluate_condition({'foo': 23, 'bar': 43}))
+        self.assertTrue(dep.evaluated_condition)
         self.assertRaises(TypeError, Dependency, 'foo', unknownattribute=42)
 
     def test_init_kwargs_string(self):