After two decades of theoretical and descriptive work, we know a great deal about the developmental course of early attachment relationships. We know considerably less about the mechanisms underlying consistency and change. Indeed, the most pressing issue in attachment theory is to explain well-replicated correlations between early care and subsequent patterns of secure base behavior, and between secure base behavior in infancy and subsequent behavior with parents and siblings, social competence, self-esteem, and behavior problems. As a step in this direction, we examine Bowlby's developmental outline, with an eye toward providing greater detail and incorporating traditional learning mechanisms into Bowlby's attachment theory.