We introduce a speci cation technique and a re nement calculus for networks of components communicating asynchronously via unbounded FIFO channels. Speci cations are formulated in a relational style. The given re nement rules allow modular system development in a step-wise, top-down manner. We employthe proposed formalism to specify and develop a so-called Min/Max Component. First an overview of the whole design process is given. Then certain steps of the development are described in detail.