This paper describes the Self-Evaluation Maintenance Model (SEM), developed by psychologists to learn more about stability and change in self-definition. An initial section discusses the situational-motivational processes assumed necessary for individuals to maintain a positive self-evaluation, i.e., a reflection process and a comparison process. Experimental tests of the model along with research results are explained. An interpretational questions, dealing with public versus private self-evaluation maintenance, is discussed. A final section discusses the SEM model in relation to other approaches to self-definition, i.e., the situational and the ontogenetic. Directions for future research are indicated. (PAS)