This chapter explores the concept of fidelity in health services evaluation, emphasizing its crucial role in ensuring effective healthcare delivery and improved outcomes for healthcare practices and patient care. Fidelity, in this context, refers to the degree to which healthcare interventions are implemented as intended. This chapter discusses how the dimensions and definitions of fidelity can differ across various conceptual frameworks. Key dimensions contributing to fidelity measures (i.e., treatment protocols, adherence, quality of delivery), which could be considered for assessing fidelity, are identified and discussed. Contextual factors are highlighted as influencers of fidelity as well as the connection between fidelity and intervention results. Practical guidance to incorporate fidelity assessment into healthcare evaluation is provided. This includes ethical considerations and trends, and challenges in fidelity evaluation.