It has long been recognized that the “part” relation is an extremely useful modeling tool. This is especially true in areas such as manufacturing, design, graphics, and document processing. In this paper, we present a comprehensive conceptual model for parts in the context of object-oriented database (OODB) systems. Our model captures the semantics and functionality of a variety of part relations with characteristics such as exclusiveness/ sharing, multi-valuedness, cardinality restriction, ordering, essentiality, dependency, and value propagation. Our notion of exclusiveness extends previous work by refinement into two kinds, inter-class and intra-class exclusiveness. Dependency in our model is permitted from both the part to the whole object, and vice versa. We also present a general mechanism for upward and downward value propagation along the part relation. Of note is the fact that we realize the part model without having to introduce any extraordinary new constructs into the underlying OODB data model. The part relation itself is represented as an object class whose instances represent the actual part connections between instances of the participating classes. By elevating the part relation to the status of a “first-class citizen,” we are following in the tradition of the ER and other semantic data models.