The practical application of the Object Constraint Language, which is part of the UML specification since version 1.1, depends crucially on the existence of adequate tool support. This paper discusses general design issues for OCL tools. It is argued that the nature of OCL will lead to a large variety of tools, applied in combination with a variety of different UML tools. Therefore, a flexible modular architecture for a UML/OCL toolset is proposed. The paper reports on the first results of an ongoing project which aims at the provision of such an OCL toolset for the public domain.