OBJECTIVES: To provide an overview of subjective and objective measures used in clinical trials to assess adherence or compliance to medical treatment in diabetes mellitus, to identify trends in the measurement of adherence/compliance, standardization, and/or tool usage frequency. METHODS: The search was performed on September 1, 2011 and was limited to articles written since 2000. The following keywords and their descriptors in Medline via OVID were used: patient compliance, adherence, clinical trials, and diabetes mellitus. Once the abstracts had been retrieved, they were carefully reviewed and either selected or rejected, according a ranking process. RESULTS: A total of 508 references were retrieved and 108 selected. Forty-eight measures were identified: 36 to assess adherence, and 12 to assess compliance. Among the adherence measures, 27 were subjective measures such as patient-reported outcome (PRO) instruments and nine, objective. The most-quoted subjective measures used were the Diabetes Self-Management Profile (DSMP) conventional or flexible regimen versions (four times); the Summary of Diabetes Self-Care Activities (SDSCA) (four times); the five-item Medication Adherence Report Scale (MARS-5) (two times); the Morisky Medication-Taking Adherence Scale (MMAS) (two times); and the Brief Medication Questionnaire (BMQ) (two times). The most-quoted objectives measures to assess adherence were the refill of repeat prescriptions (cited six times) and pill cap monitoring systems (five times). As for the assessment of compliance, only two were subjective measures and ten were objective. The PRO measures retrieved were the Chronic Disease Compliance Instrument and a Medication Compliance Scale. The most frequently used objective measure was the evaluation of return of study drugs. CONCLUSIONS: Diabetes is a challenging disease to manage successfully and the associated costs of non-adherence are high. This review shows a lack of standardization/consistency in the measurement of adherence/compliance in diabetes. More research is needed on what are the “best” tools to use to measure adherence-related behavior in diabetes.