Assessing treatment fidelity is a core methodological consideration in the study of treatment outcome; it influences both the degree to which changes can be attributed to the intervention and the ability to replicate and disseminate the intervention. Efforts to increase access to evidence-based psychological treatments are receiving unprecedented support; but pressures exist to adapt treatments to service settings, running the risk of compromising fidelity. However, little evidence is available to inform the necessary conditions for the transportation of interventions to service provision settings, and the degree to which fidelity is even evaluated or emphasized in dissemination and implementation programs varies dramatically. Moreover, adaptation is associated with several benefits for dissemination efforts and may address relevant barriers to adoption. A particularly promising strategy for maximizing the benefits of both fidelity and adaptation is the use of transdiagnostic interventions. Such treatments allow for greater flexibility of the pacing and content of treatment, while still providing structure to facilitate testing and replication. Preliminary evidence supports the efficacy of this strategy, which may be particularly conducive to dissemination into service provision settings. At this time, further research is needed to evaluate the relationships among fidelity, adaptation, and outcome, and to determine the potential for transdiagnostic treatments to facilitate dissemination.