Fidelity measurement is critical for testing the effectiveness and implementation in practice of psychosocial interventions. Adherence is a critical component of fidelity. The purposes of this review were to catalogue adherence measurement methods and assess existing evidence for the valid and reliable use of the scores that they generate and the feasibility of their use in routine care settings. A systematic literature search identified articles published between 1980–2008 reporting studies of evidence-based psychosocial treatments for child or adult mental health problems, including mention of adherence or fidelity assessment. Coders abstracted data on the measurement methods and clinical contexts of their use. Three hundred forty-one articles were reviewed, in which 249 unique adherence measurement methods were identified. These methods assessed many treatment models, although more than half (59%) assessed cognitive behavioral treatments. The measurement methods were used in studies with diverse clientele and clinicians. The majority (71.5%) of methods were observational. Information about psychometric properties was reported for 35% of the measurement methods, but adherence-outcomes relationships were reported for only 10%. Approximately 1/3 of the measures were used in community-based settings. Many adherence measurement methods have been used in treatment research; however, little reliability and validity evidence exists for the use of these methods. That some methods were used in routine care settings suggests the feasibility of their use in practice; however, information about the operational details of measurement, scoring, and reporting is sorely needed to inform and evaluate strategies to embed fidelity measurement in implementation support and monitoring systems.