The exchange of models among different modeling tools ever more becomes an important prerequisite for effective software development processes. Due to a lack of interoperability, however, it is often difficult to use tools in combination, thus the potential of model-driven software development cannot be fully exploited. This paper proposes ModelCVS, a system which enables tool integration through transparent transformation of models between different tools´ modeling languages expressed as MOF-based metamodels. ModelCVS provides versioning capabilities exploiting the rich syntax and semantics of models. Concurrent development is enabled by storing and versioning software artifacts that clients can access by a check-in/check-out mechanism, similar to a traditional CVS server. Semantic technologies in terms of ontologies are used together with a knowledge base to store machine- readable, tool integration relevant information, thus allowing to minimize repetitive effort and partly automate the integration process.