  A C*-dynamical system is said to have the ideal separation property if every
ideal in the corresponding crossed product arises from an invariant ideal in
the C*-algebra. In this paper we characterize this property for unital
C*-dynamical systems over discrete groups. To every C*-dynamical system we
associate a "twisted" partial C*-dynamical system that encodes much of the
structure of the action. This system can often be "untwisted," for example when
the algebra is commutative, or when the algebra is prime and a certain specific
subgroup has vanishing Mackey obstruction. In this case, we obtain relatively
simple necessary and sufficient conditions for the ideal separation property. A
key idea is a notion of noncommutative boundary for a C*-dynamical system that
generalizes Furstenberg's notion of topological boundary for a group.
