Realist evaluation has, over the past two decades, become a widely used approach in evaluation. The cornerstone of realist evaluation is to answer the question: What works, for whom, under what circumstances, and why. This is accomplished by explicating the causal mechanisms that, within a particular context, generate the outcomes of interest. Despite the central role of mechanisms in realist evaluation, systematic knowledge about how the term mechanism is conceptualized and operationalized is limited. The aim of the present chapter is to examine how mechanisms are defined and applied in realist evaluations. Informed by the findings of the review, further conceptual and practical developments for future applications of mechanisms in realist evaluation are considered.