OBJECTIVES: This study reviews adherence metrics for Phase IV studies. METHODS: We conducted a review of adherence metrics in the public domain. We critically appraised these metrics for use in Phase IV studies. RESULTS: We identified 70 unique self-report measures of adherence. One quarter (26%) were generic and the remaining were disease specific. Instrument length ranged from one to 78 items. One third (34%) only measured adherence behaviors, 37% only measured beliefs and attitudes, and 29% measured both. Just over one quarter (29%) were developed using a conceptual framework. One-fifth (21%) involved qualitative patient input during item generation or pretesting. Just over one-half (57%) had evidence of internal-consistency reliability, and far fewer had evidence of test-retest reliability (23%). One half (50%) had evidence of validity vis á vis other self-report measures, 23% vis á vis other adherence metrics, and 19% vis á vis clinical criteria. Few had evidence of predictive (24%) or postdictive (13%) validity. CONCLUSIONS: Few adherence measures have been developed with true patient-centerednesss. There has been no standardization of the content of adherence behaviors or  beliefs. Instrument validation has been inconsistent in its breadth and depth.  Because of the importance of medication adherence to payers, providers, pharmacies, and pharmaceutical companies, the time seems opportune to conceptualize, develop, and validate a generic adherence measure that can be used in Phase IV studies across different disease and patient groups. Standardization of content would allow for the assessment of adherence behaviors and beliefs between and across existing and novel therapies. There should be a minimum set of adherence concepts that apply across therapeutic areas. The new measure should be developed with patient input (concept elicitation) and verified as to its comprehension and relevance using cognitive debriefing. The scientific basis of medication adherence would be advanced through the development and validation of a standardized generic measure that assesses adherence behaviors and beliefs.