The purpose of this dissertation is to further develop the concept of identity development in identity theory. Identity development is the process of change that occurs in identities during which the meanings that are contained in identities become less inconsistent. Individuals will work to reduce the amount of inconsistencies between their identity meanings so that they may verify their identities in situations. Identities will be verified more frequently the more they are developed. I suggest that the hierarchical control system in identity theory, which is a complex model that explains how multiple identities interact in situations, can model identity development among multiple identities. Using the hierarchical control system, I attempt to integrate the concept of identity development into established theory in identity theory.I also suggest that there are three phases of identity development: the pre-formative, formative, and transformative phases. The pre-formative phase of identity development occurs as identities begin to emerge from having not existed. Little is known in identity theory with respect to how identities emerge, but this early phase in identity development is important to understand from where identities and identity meanings derive. The formative phase takes place after identities have already emerged and begin to interact with one another in situations. The formative phase explains how the meanings that are contained in identities that define who persons are in social positions, such as the categorical/group and role identities, will influence more abstract person identity meanings that are attached to the individual. Finally, the transformative phase explains how person identity meanings influence categorical/group and role identity meanings. A focus on identity development advances identity theory in several ways, which I discuss in the conclusion of this dissertation.