OBJECTIVE: To propose a definition of compliance/adherence and persistence which would be widely agreed and useful in providing consistency for clinical, health policy and clinical practice research. METHODS: The “Issues and Definitions Working Group” of the Medication Compliance Special Interest Group undertook to review definitions that could be used for medication compliance/adherence and persistence. Broad definitions were presented at an ISPOR workshop in 2003 and revised accordingly. These definitions were then placed on the ISPOR website and all members were given an opportunity to comment and vote on definitions in December 2004. Although consensus was reached for the compliance and persistence definitions, many key issues related to these definitions required resolution. At the Annual Meeting 2005, a workshop was held to discuss the issues around operationalizing compliance/adherence and persistence definitions. Comments were incorporated and the final operational definitions posted for consensus on the website.RESULTS: Medication Compliance (Synonym: Adherence)is the extent to which a patient acts in accordance with the prescribed interval and dose as well as dosing regimen. The unit of measure for compliance is administered doses per defined period of time, reported as a proportion (%) of prescribed doses (D) taken at the prescribed time interval (T) as measured by the period of time, i.e., % of TD, measured by percentage. CONCLUSION: Medication Persistence is the duration of time patient remains on treatment i.e. accumulation of time from initiation to discontinuation of therapy, where patient is defined as a discontinuer if medications were not taken within a predefined permissible time gap.