Evaluators are often called upon to assess the effects of programs. To assess a program effect, evaluators need a clear understanding of how a program effect is defined. Arguably, the most widely used definition of a program effect is the counterfactual one. According to the counterfactual definition, a program effect is the difference between what happened after the program was implemented and what would have happened if the program had not been implemented, but everything else had been the same. Such a definition is often said to be linked to the use of quantitative methods. But the definition can be used just as effectively with qualitative methods. To demonstrate its broad applicability in both qualitative and quantitative research, I show how the counterfactual definition undergirds seven common approaches to assessing effects. It is not clear how any alternative to the counterfactual definition is as generally applicable as the counterfactual definition.