The objective of this report is to provide an overview of current knowledge related to the water sector1 across Small Island Developing States (SIDS) of the Pacific region. SIDS face a specific set of water, sanitation, and hygiene (WASH) challenges related to the extreme fragility of their water resources to anthropogenic impacts and climate change, the unique features of their water cycles, lack of scale, and isolation. Many of the modern water management paradigms applied elsewhere need careful reconsideration in that context. Consolidating a review of current knowledge, approaches, and best practices across the region can therefore be of high value to stimulate new thinking and prevent duplication of efforts. The reader will find captured in this report the findings and recommendations of development practitioners, researchers, policy makers who have left their mark over the past decades on the water, sanitation and hygiene sector in the Pacific region. Those can in turn help inform the design of future water, sanitation and hygiene development projects and programs, and the scoping of academic work. This rapid study does not claim to have captured in a fully comprehensive manner the entire scope of knowledge on the subject, but with more than 500 relevant publications identified and reviewed, it most likely offers a reasonable overview of the current state of knowledge. To complement and put in perspective the analysis on Pacific SIDS, the study included reflections on of practice and knowledge in non-Pacific SIDS. The first section of the report provides a summary of findings in terms of key knowledge areas and gaps, and proposes recommendations for future analytical work in the water sector in the Pacific. The following sections provide a more detailed review of current knowledge across seven key challenges identified as particularly critical in Pacific SIDS context: (i) environmental variability and change; (ii) sociocultural aspects; (iii) isolation and small scale; (iv) uncontrolled urbanization; (v) service delivery models; (vi) sector coordination, integration, and capacity; and (vii) data, information, and knowledge. Each topic is covered by a dedicated section describing the nature of the challenge, the identified ways forward, and knowledge gaps. Appendix A provides a description of the methodology used for this study.