The type systems of most object-orientecl database syst ems (OODBSS) descend from traditional object-oriented programming lan gna,ges. While these systems can capture many interesting relationships among entities, such as classification ancf snbtyping, they do not typically allow an object, to change type, and they only partially support the mode]ling of objects that have many types. However, such characteristics are particularly common among the very entities (e. g., people) that these systems are intended to model. We introduce aspects, a new mechanism designecf to meet, these modelling requirements within the framework of a strongly-typed 00DBS. An aspect extends an existing object with new state and new behavior while maintaining the same object identity. In addition to the modelling of roles, aspects have other interesting applications, such as encapsulating the result, of a query. © 1991, ACM. All rights reserved.