Adherence is an important element in the medical field since it is thought to be the link between treatment and outcomes. Adherence to medications has been extensively researched and it is evident that non-adherence is common across most disease states. These studies vary by the conceptual definitions of adherence behavior and by the research paradigms. The objectives of this presentation are to review the conceptual definitions used in adherence research and to review the theoretical frameworks used to explain mediation adherence behavior. Compliance, adherence, and concordance are used interchangeably in the medical, health behavior, and pharmacy literature. It is important to compare and contrast these terms to study specific health behavior. These terms reflect different philosophies of medicine with respect to the provider-patient relationship. Conceptually, adherence, compliance and concordance differ in the amount of patient involvement and participation, that may be depicted along a continuum of patient involvement- with compliance depicting no patient involvement, concordance depicting patients as being equal partners in their treatment and adherence lying somewhere in between. Consistent use of these concepts will move the science toward understanding specific patient behavior and its antecedents. Much of the adherence research published in the medical and pharmacy journals does not include a theoretical framework. The non-theoretical approach to adherence research is partly to blame for the lack conceptual clarity and underscores the need to incorporate a theoretical basis in adherence research. Prominent theories in adherence research include expectancy-values models like the health belief model, the transtheoretical model, and the self-regulation theory. Other promising models include the medication adherence model, the interaction model of client behavior and the therapeutic decision model. The strengths and weaknesses of each are presented. Finally, recommendations for researchers of medication adherence include using a theoretical framework and conducting longitudinal studies are provided.