This chapter describes the evolution and expanded uses of evaluability assessment (EA) from its 1970s beginnings as a pre-evaluation activity for determining if a program is ready for productive outcome evaluation to its flexible use across a variety of health service contexts. Current uses include, for example, formative evaluation, evaluation capacity building, and maximizing limited evaluation resources. The authors outline EA as a threefold check on program plausibility, program reality, and professional evaluation standards and provide examples from a brief review of recent EA publications in health services. They end with related recommendations for unlocking the potential of EA to improve health service evaluation and programs.