Non-adherence to medications is a prevalent and persistent healthcare problem, particularly for patients with a chronic disorder. Researchers have endeavoured to address poor adherence for the past five decades resulting in the accumulation of a vast body of literature. Despite the enormity of research conducted, interventions to date have neither been cost-effective nor predictably clinically effective in enhancing medication adherence. Though concerning, such contemporary information serves to refocus attention on the adequacy of knowledge regarding the factors influencing medication non-adherence. Although little consensus exists regarding the optimal categorisation of these influencing factors, increasingly, the broad and 'all encompassing' categorisation of intentional and unintentional factors is being used to account for patient medication-taking behaviours and actions. An extensive review of the related literature provides the basis for a critical discussion on the value and comprehensiveness of this current classification in guiding future adherence research and consequent clinical interventions. An appraisal of this categorisation is important if decisions regarding interventions are not to be made in a vacuum of insufficient understanding, which would result in the continued ineffective use and distribution of valuable resources to combat non-adherence.