In practice, concurrent programming systems based on message passing are often instantiations of the actor model. A popular implementation of this form of concurrency is the Erlang programming language. Erlang supports massively concurrent systems such as telephone exchanges by providing very lightweight concurrent processes. On mainstream platforms such as the JVM, an equally attractive implementation was as yet missing. Their standard concurrency constructs, shared-memory threads with locks, suffer from high initialization and context-switching overhead as well as high memory consumption. Therefore, the interleaving of independent computations is often modelled in an event-driven style on these platforms. However, programming in an explicitly event-driven style is complicated and error-prone, because it involves an inversion of control. In this paper we present an abstraction of actors that combines the benefits of thread-based and event-based concurrency. Threads support blocking operations such as system I/O, and can be executed on multiple processor cores in parallel. Event-based computation, on the other hand, is more lightweight and scales to large numbers of actors. We also present a set of combinators that allows a flexible composition of these actors.