Although experimental studies are described as the "gold standard" for assessing the effectiveness of a program in changing outcomes, in some cases, quasi-experimental studies may be more feasible or appropriate. Many types of quasi-experimental studies are possible. For example, an implementation study can provide valuable information on whether, how, and for whom services are provided, and an outcome monitoring study can provide early information on whether outcomes are changing as expected. This brief describes varied types of quasi-experimental evaluations and the circumstances under which they are useful. (Contains 5 notes.)