. Propositional Dynamic Logics (PDL's) provide a suitable formal framework for modeling actions and reasoning about them. However, the basic language of PDL's lack several features that are important for a sophisticated treatment of actions. In this paper, we present a new logic that is obtained by enriching the basic PDL with powerful modeling constructs that allow us to represent determinism and nondeterminisms, concurrency, hierarchies, mutual exclusion, backward execution, and non-execution of actions. We demonstrate, by means of examples, the expressive power of the formalism. In particular, we show that although nonmonotonicity is not generally captured by PDL's, our logic is perfectly suited for exploiting monotonic solutions to the frame problem. Finally, we establish that the proposed formalism is decidable, and that the basic reasoning problems are EXPTIME-complete. 1 Introduction Propositional Dynamic Logics (PDL's) are modal logics for describing and reasoning about system..