The Entity-Relationship (ER) model (Chen, 1976) is frequently used for the specification of conceptual database schemas. Entity types and relationship types are the building blocks provided by this data model. A major objective of conceptual database design (see e.g. Batini, Ceri, & Navathe, 1992) is to define entity types and relationship types in such a way that they represent meaningful units of information with respect to the semantics of the modeled application domain. Object types in object-oriented data models are comparable to entity types as far as structural aspects are concerned. However, the ER model explicitly supports relationship types, whereas different approaches are found in object-oriented data models: On the one hand, relationship types may be represented by reference attributes, which means that they are part of the object type itself. This approach, for example, is often found in programming languages. On the other hand, they may be represented explicitly by means of a separate concept as in the ER model. In the latter case there is a close resemblance between entity types and object types. Object types represent sets of objects which are of relevance in the application domain, while relationship types represent relationships between objects.