and, conversely, the construction of composite objects out of individual parts have long been recognized as ubiquitous and essential mechanisms involving abstraction. This applies, in particular, in areas such a s CAD, manufacturing, software development, and computer graphics. Although the part-of relationship is distinguished in object-oriented modeling techniques, it ranks far behind the concept of generalization/specialization and a rigorous de nition of its semantics is still missing. In this paper we rst show i n w h i c h w ays a shift in emphasis on the part-of relationship leads to analysis and design models that are easier to understand and to maintain. We then investigate the properties of part-of relationships in order to de ne their semantics. This is achieved by means of a categorization of part-of relationships and by associating semantic constraints with individual categories. We further suggest a precise and, compared with existing techniques, less redundant speci cation of constraints accompanying part-of categories based on the degree of exclusiveness and dependence of parts on composite objects. Although the approach appears generally applicable, the object-oriented Uni ed Modeling Language (UML) is used to present our ndings. Several examples demonstrate the applicability of the categories introduced. Index terms: Conceptual modeling, object-oriented analysis, object-oriented design, part-of relationship, part-of relationship categories, partonomy, aggregation, semantic constraints, object lifecycle. OO-terms categories of characterization, method used part-of relationship comment, example Booch has-relationship by-value aggregation part p is a part of the state of whole w, aggregation by-reference aggregation attributes are considered parts OMT part-of-relationship not distingusished attributes are not considered parts, operation aggregation propagation to be considered Jacobson consists-of relationship not distinguished an aggregate is composed of other objects, its parts composition partonomies are considered Martin/ composed-of relationship mutable composition composition is the act or result of forming Odell composition immutable composition an object from its component p a r t s Embley part-of-relationship part-of distinguished attributes are considered parts, e.g., aggregation from member-of Ordernumber part-of Order but: Student m e m ber-of StudentClub UML whole-part relationship by-value aggregation composition indicates that the lifetime of the parts version 0.9 aggregation or composition is dependent on the lifetime of the whole and 1.0 by-reference aggregation by-value aggregation is semantically equivalent t o an attribute properties: transitivity, a n tisymmetry