Formal derivation refers to a family of design techniques that entail the development of programs which are guaranteed to be correct by construction. Only limited industrial use of such techniques (e.g., UNITY-style specification refinement) has been reported in the literature, and there is a great need for methodological developments aimed at facilitating their application to complex problems. This article examines the formal specification and design of a message router in an attempt to identify those methodological elements that are likely to contribute to successful industrial uses of program derivation. Although the message router cannot be characterized as being industrial grade, it is a sophisticated problem that poses significant specification and design challenges—its apparent simplicity is rather deceiving. The main body of the article consists of a complete formal specification of the router and a series of successive refinements that eventually lead to an immediate construction of a correct UNITY program. Each refinement is accompanied by its design rationale and is explained in a manner accessible to a broad audience. We use this example to make the case that program derivation provides a good basis for introducing rigor in the design strategy, regardless of the degrees of formality one is willing to consider.