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

Noncommutative boundaries and the ideal structure of reduced crossed products