Treatment fidelity refers to the extent to which an intervention is implemented as planned. If researchers do not assess and report treatment fidelity, or if treatment fidelity is shown to be low, findings from intervention studies are difficult to interpret, because the intervention may not have been implemented as planned. In this article, our aim is to inform research consumers by discussing treatment fidelity and its primary dimensions, providing guidelines for interpreting treatment fidelity, considering implications of treatment fidelity for research and practice, and illustrating how fidelity is reported in two recent studies. Our take-home message is that when one is interpreting intervention studies, it is important to consider whether the interventions were applied as planned, or with fidelity.