Mental health intervention research requires clear and accurate specification of treatment conditions in intervention studies. Measures are increasingly available for community-based interventions for persons with serious mental illnesses. Measures must go beyond structural features to assess critical processes in interventions. They must also balance effectiveness, or adequate coverage of active treatment elements, with efficiency, or the degree to which measures may be used cost-effectively. The context of their use is changing with the emergence of new frameworks for implementation research and quality improvement. To illustrate a range of approaches, this article describes four recently developed fidelity measures: Cognitive Therapy for Psychosis Adherence Scale, Strengths Model Fidelity Scale, Illness Management and Recovery Program Fidelity Scale, and Tool for Measurement of ACT. The fidelity measures assess interventions in a range of treatment contexts from dyads to teams. Each measure focuses assessment resources on critical elements. Each has demonstrated coverage of its target intervention and satisfactory psychometric properties and is related to outcomes. Measures have been used for training, quality improvement, or certification. They assess domains and have uses beyond their nominal position in implementation and quality frameworks. This review of recent fidelity measures indicates that process components in community-based interventions can be effectively assessed. Omission of elements assessing potentially critical active treatment components poses risk to both research and practice until there is evidence to demonstrate that they are nonessential. Further development of fidelity measurement theory and approaches should proceed in conjunction with development of theory and methods in implementation science.