To review the definitions and methods for measuring medication persistency, and to propose a uniform definition of and calculation for persistency using pharmacy claims data. Literature review. A MEDLINE search (1966 to present) was performed to identify articles detailing a definition or method of persistency measurement based on automated pharmacy data. Articles were screened for relevance by title and abstract. References from identified articles were used to expand the search results. The concept behind medication persistency measurement is to capture the amount of time that an individual remains on chronic drug therapy. The methods to calculate medication persistency can be classified into 1 of 3 categories: (1) Persistency as a function of the medication possession ratio; (2) persistency as a function of medication availability at a fixed point in time; and (3) persistency as a function of the gaps between refills. The common goal of all persistency measures should be to reflect the continuity of medication usage and to capture the timeliness and the frequency of refilling. The measurement of persistency as a function of the gaps between refills provides the best assessment of refill compliance across a variety of medication and disease states and lends itself to the well-established measurements of survival analysis.