Over the past 2 decades, the number of after-school programs (ASP) and the number of students attending ASPs has markedly increased. Although several reviews and meta-analyses have examined the outcomes of ASPs, ASP intervention study reviews have not specifically examined intervention fidelity. Establishing intervention fidelity is critically important to interpreting the effects, or lack thereof, of interventions. Given the popularity of ASPs and the growing body of intervention research resulting in ambiguous findings, it seems prudent to examine whether investigators have attended to fidelity. The purpose of this systematic review was to examine whether and to what extent researchers addressed intervention fidelity in ASP intervention research. The following research questions guided this review: (1) What proportion of after-school intervention studies report key components of fidelity (i.e., strategies to enhance fidelity, measure fidelity, and use fidelity data)?; and (2) Does the presence of fidelity measurement differ by study or intervention characteristics? Fifty-five studies (15 RCT and 40 QED) were included in this review. Published and unpublished studies conducted between 1980 and 2012 were eligible for inclusion if they examined the effects of an ASP on social, emotional, behavioral, or academic outcomes with primary or secondary students using a randomized or quasi-experimental research design. This study examined the extent to which fidelity was addressed in outcome research of after-school program interventions, defined as "an organized program offering one or more activities that: (1) occurred during at least part of the school year; (2) happened outside of normal school hours; and (3) was supervised by adults". 40 of the 55 studies attended to at least some aspects of intervention fidelity; however, the extent to which those studies engaged in various aspects of intervention fidelity varied. While the majority of studies attended to at least one aspect of intervention fidelity, the use of multiple fidelity components was much less frequent. About half of the studies incorporated two components, 31% incorporated three components, 18% incorporated four components, and just 15% incorporated at least five components. No studies incorporated six or more fidelity components. Tables and figures are appended.