The State of the Science in Spirituality and Palliative Care was convened to address the current landscape of research at the intersection of spirituality and palliative care and to identify critical next steps to advance this field of inquiry. Part I of the SOS-SPC two-part series focuses on questions of 1) What is spirituality? 2) What methodological and measurement issues are most salient for research in palliative care? And 3) What is the evidence relating spirituality and health outcomes? After describing current evidence we make recommendations for future research in each of the three areas of focus. Results show wide variance in the ways spirituality is operationalized and the need for definition and conceptual clarity in research in spirituality. Furthermore, the field would benefit from hypothesis-driven outcomes research based on a priori specification of the spiritual dimensions under investigation and their longitudinal relationship with key palliative outcomes, the use of validated measures of predictors and outcomes, and rigorous assessment of potential confounding variables. Finally, results highlight the need for research in more diverse populations. J Pain Symptom Manage 2017;54:428e440. Published by Elsevier Inc. on behalf of American Academy of Hospice and Palliative Medicine.