Languages like F#, C#, and recently also Scala, provide “async” extensions which aim to make asynchronous programming easier by avoiding an inversion of control that is inherent in traditional callback-based programming models (for the purpose of this paper called the “Async” model). This paper outlines a novel approach to integrate the Async model with observable streams of the Reactive Extensions model which is best-known from the .NET platform, and of which popular implementations exist for Java, Ruby, and other widespread languages. We outline the translation of “Reactive Async” programs to efficient state machines, in a way that generalizes the state machine translation of regular Async programs. Finally, we sketch a formalization of the Reactive Async model in terms of a small-step operational semantics.