UML Version 1.0 has been submitted as proposed OOAD standard to OMG by Rational in January 1997 [BJR97]. Due to considerable weaknesses, this version could not become the expected standard and the work on a new version had to be started. I actually do not expect that this version will be the definitive standard. But it can be expected that UML version x.y will be the forthcoming OOAD standard of OMG as there is no serious competitor in sight. Considering the popularity UML has already gained during the last year, and considering the consortium that stands behind UML, there is no doubt that this standard will clearly dominate the market. The main purpose of this OOAD standard is to ensure interoperability between different OOAD tools and the various methods they support. In a first step, the interoperability is restricted to a transfer of models between the tools.