Principles-focused evaluation (PFE) is an emergent approach to evaluation in health care. It is useful for evaluating health services where principles guide implementations and adaptations across different contexts, in complex situations, and when preparing to scale a program or intervention up or out. Unlike evaluation approaches emphasizing standardized performance indicators, outputs, and outcomes, PFE examines the meaningfulness of principles guiding the program, how the principles are enacted, and associated outcomes. In this chapter, we discuss the process of conducting a PFE, including developing evaluation questions and effectiveness principles and designing methods to collect, analyze, and interpret data. We describe characteristics of health services that make them amenable to PFE and those that do not, as well as the strengths and limitations of this evaluation approach.