Due to its rich set of modeling concepts and its broad application spectrum the Unified Modeling Language (UML) has become widely accepted for modeling many aspects of software systems. Since UML is not related to any particular design method, each software development project has to establish its own modeling guidelines. Hence, tool support is needed for guiding the developer throughout the modeling process and for enforcing project-related integrity of UML models. In this chapter, we present our approach for enforcing guidelines in UML-based software development processes. For managing UML models, we implemented a UML repository on top of an object-relational database management system (ORDBMS). Guidelines are expressed as OCL constraints and are enforced either automatically, i. e., by the UML repository, or on user demand. For this purpose, we take advantage of ORDBMS query facilities for checking guidelines by automated mapping of OCL constraints to SQL expressions.