This essay is a companion document to a statistical study of student dropouts conducted by the San Diego City Schools. Various definitions of dropout are presented, as well as a discussion of types of dropout or attrition rates: (1) annual dropout rate; (2) enrollment ratio dropout rate; (3) cumulative tracking dropout rate; and (4) estimated cumulative dropout rate. Based on a review of the literature, demographic, school- related, and other characteristics of dropouts and factors leading to the decision to drop out are described. A systems theory perspective is used to identify three general classes of factors that affect a school system's dropout rate: (1) characteristics of the students entering the school system (input factors); (2) characteristics, policies, and programs of the system itself (throughput factors); and (3) economic and social conditions of the surrounding community, state, and nation (environmental factors). To establish the foundation for a theory of retention, four types of dropout/attrition explanation are distinguished, made up of two kinds--academic and empowering, and two levels--individual and school system. Within a systems perspective, an empowering theory of student retention is proposed. According to this model, a school system can increase its student retention rate by successfully delivering communications of personal support to each student and enabling each student to successfully meet meaningful academic challenges. Forty-two references are provided. (MW)