In outcome research, fidelity has typically been described as the ability to show that manipulation of the independent variable has occurred as designed. Assessment of prevention program fidelity—that is, the delivery of a manualized program as prescribed by the developer—is useful to providers in determining the level of quality of a given implementation and to offer assurances to providers that services are, in fact, impacting the intended populations. This entry will discuss salient issues regarding fidelity of prevention programming, including the evaluation and measurement of fidelity, elements that may serve to enhance fidelity, and the tension between fidelity and adaptation of programs to specific needs and populations.