This paper presents a state-of-art review of empirical research on object-oriented (OO) design. Many claims about the cognitive benefits of the OO paradigm have been made by its advocates. These claims concern the ease of designing and reusing software at the individual level as well as the benefits of this paradigm at the team level. Since these claims are cognitive in nature, its seems important to assess them empirically. After a brief presentation of the main concepts of the OO paradigm, the claims about the superiority of OO design are outlined. The core of this paper consists of a review of empirical studies of OOD. We first discuss results concerning OOD by individuals. On the basis of empirical work, we (1) analyse the design activity of novice OO designers, (2) compare OO design with procedural design and, (3) discuss a typology of problems relevant for the OO approach. Then we assess the claims about naturalness and ease of OO design. The next part discusses results on OO software reuse. On the basis of empirical work, we (1) compare reuse in the OO versus procedural paradigm, (2) discuss the potential for OO software reuse and (3) analyse reuse activity in the OO paradigm. Then we assess claims on reusability. The final part reviews empirical work on OO design by teams. We present results on communication, coordination, knowledge dissemination and interactions with clients. Then we assess claims about OOD at the software design team level. In a general conclusion, we discuss the limitations of these studies and give some directions for future research.