It is well established that pharmacotherapy is fundamental to disease and symptom management. Adherence to medication regimens is therefore essential for improved patient health outcomes. In reality, however, many patients experience difficulty with medication taking resulting in suboptimal adherence. Given the consequences of this pervasive problem, non-adherence is increasingly recognized as one of the leading challenges that professionals face in contemporary health care. As health-care professionals, nurses have an important role in combating this problem. This paper therefore examines the literature surrounding medication adherence for the purposes of enhancing professional knowledge and practice in this area. Specifically, the impact of poor adherence from the patient's, health-care professional and health-care system perspective is detailed to highlight the significance of this issue. The underlying philosophical attributes of adherence and the interchangeable concepts of compliance and concordance are explored, with the implications of the meanings attributed to these concepts for nursing practice and research considered.