Retrieval of software components from a library relies on techniques for matching user requirements against library component interfaces. In this paper we introduce a number of techniques for matching formally specified, state-based modules. These techniques will form the basis for retrieval tool support. The techniques described in this paper build on existing specification matching methods, based on individual functions, specified using pre- and post-conditions. We begin by defining a basic module matching technique, based on matching the individual units within a module. We consider variations of this technique that take into account two important features of modules: the visibility of module entities; and the use of state invariants. An advanced technique, based on data refinement and the use of coupling invariants, is also described.