Fidelity refers to the demonstration that an experimental manipulation is conducted as planned. In outcome research, an intervention can be said to satisfy fidelity requirements if it can be shown that each of its components is delivered in a comparable manner to all participants and is true to the theory and goals underlying the research. Demonstrating the fidelity of an intervention is a key methodologic requirement of any sound prevention trial. This paper summarizes key conceptual and methodologic issues associated with intervention fidelity, and describes the steps taken to promote fidelity in EARLY ALLIANCE, a large-scale prevention trial currently testing the effectiveness of family, peer, and school interventions to promote competence and reduce risk for conduct disorder, substance abuse, and school failure. The paper presents preliminary results (Trial Year 1) that demonstrate content and process fidelity for two of these interventions, and discusses how the EARLY ALLIANCE methodology may be generalized to address fidelity issues in other prevention studies.