Abstract : This paper is placed in the context of large scale distributed programming, providing a programming model based on asynchronous components. It focuses on the semantics of asynchronous invocations and component synchronisation. Our model is precise enough to enable the specification of a formal semantics. A variant of this model has been implemented, together with tools for managing components. This paper explains why we consider that our component model is efficient and provides a convenient programming model. We show how futures play a major role for such asynchronous components, and provide a reduction semantics for the component model. This reduction semantics has been specified in the Isabelle theorem prover, and will be used to prove properties on the component model and its implementations.