Adherence to a prescribed medication regimen is influenced not only by characteristics of the individual patient, but also by factors within the patient's environment, or so-called system level factors. Until now, however, health care system factors have received relatively little attention in explaining medication nonadherence. Ecological models might serve as a framework to help explain the influence of health care system factors on patient behavior (e.g., adherence). In an ecological model, different levels of factors influence patients' behavior, i.e. factors at the patient-level, micro- (provider and social support), meso- (health care organization), and macro (health policy) -levels. In order to understand medication adherence and implement interventions to improve medication adherence, factors at these different levels should be taking into consideration. This paper describes an ecological model compromised of the most important factors at the patient-, micro-, meso- and macro-levels.