One of the basic activities within the software development process is the performing of model transformations. This work presents two techniques for realizing transformations of models expressed in the unified modeling language (UML). The techniques are in line with established approaches to systematically defining UML model transformations with object constraint language (OCL). Practical aspects of the definition and the implementation process are emphasized, followed by remarks on how the transformations can be used within the model driven architecture (MDA) approach to development of software systems. Advantages of the approach are highlighted.