Traditionally, evaluation has primarily tried to answer the question "Does a program, service, or policy work?" Recently, more attention is given to questions about variation in program effects and the mechanisms through which program effects occur. Addressing these kinds of questions requires moving beyond assessing average program impacts and into understanding what is inside the "black box" of a program, service, or policy. Across a number of disciplines and policy fields, significant improvements and innovations in research design, measurement, and analysis have occurred in recent years, improving the ability of evaluators to unpack the black box. This article serves to introduce the following special forum featuring papers that provide insights into various techniques that can be used to shine light into the black box of programs, services, and policies.