Over the past decade, the Patient-Centered Outcomes Research Institute (PCORI) funded multiple large-scale, comparative effectiveness clinical trials evaluating palliative care and advance care planning interventions. These are complex multicomponent interventions that need robust but flexible fidelity monitoring. Fidelity is necessary to maintain both internal and external validity within palliative care intervention research and to ultimately evaluate the real-world impact of high-quality interventions. Different trials not only took varying approaches to fidelity monitoring but also uncovered both unique and common challenges and facilitators. This article summarizes 8 of these trials and highlights approaches, adaptations, barriers, and facilitators for intervention fidelity monitoring. Identifying and delivering core elements while simultaneously allowing adaptations of noncore elements is a vital part of fidelity monitoring. Dissemination of such experiences can inform both future palliative care research as well as ongoing implementation of palliative care and advance care planning interventions across diverse clinical practices. Adoption of rigorous intervention fidelity methods is critical to advancing the science and reproducibility of palliative care interventions.