Designing reusable software modules can be extremely difficult.The design must be balanced between being general enough to address the needs of a wide range of clients and being focused enough to truly satisfy the requirements of each specific client One area where it can be particularly difficult to strike this balance is in the implementation strategy of the module.The problem is that generalpurpose implementation strategies, tuned for a wide range of clients, aren't necessarily optimal for each specific client-this is especially an issue for modules that are intended to be reusable and yet provide high-performance.An examination of existing software systems shows that an increasingly important technique for handling this problem is to design the module's interface in such a way that the client can assist or participate in the selection of the module's implementation strategy.We call this approach open implementation.When designing the interface to a module that allows its clients some control over its implementation strategy, it is important to retain, as much as possible, the advantages of traditional closed implementation modules.This paper explores issues in the design of interfaces to open implementation modules.We identify key design choices, and present guidelines for deciding which choices are likely to work best in particular situations.