An accurate assessment of fidelity, combined with a high degree of fidelity to the intervention, is critical to the reliability, validity, replicability, and scale-up of the results of an intervention research study. However, extant measures of fidelity are infrequently applicable to the program or intervention being studied, and the literature lacks guidance on the specific process of developing a system to measure fidelity in a manualized intervention. This article describes a five-step process to define the scope, identify components, develop tools, monitor fidelity, and analyze outcomes to develop a comprehensive fidelity measurement system for an intervention. The process describes the components, measures and key decisions that form a comprehensive fidelity measurement system. In addition, the process is illustrated by a case study of the development of a fidelity measurement system for a research study testing Pathways Triple P, a behavioral parent-training program, with a population of child welfare-involved families. Pathways Triple P is a common, manualized intervention and the process described in this article can be generalized to other manualized interventions. The implications and requirements for accurately assessing and monitoring fidelity in research studies and practice are discussed.