The Pacific-Australia Climate Change Science and Adaptation Planning (PACCSAP) Water Security in Tuvalu project completed a comprehensive Cost Benefit Analysis (CBA) of potential water security interventions in Funafuti and Vaitupu, two of the major islands in Tuvalu. This booklet presents the results of the CBA, and in doing so, aims to support smart, data-driven investment decisions that are essential for resource-challenged nations like Tuvalu. This publication was produced under the Pacific-Australia Climate Change Science Adaptation Planning (PACCSAP) programme.