Water insecurity-the lack of adequate and safe water for a healthy and productive life-is one of the greatest threats facing humans in the coming century. By 2030, half of the world is expected to be living in water-stressed conditions, given current climate change scenarios. A key goal of the UN Water Action Decade and Sustainable Development Goal 6 is to improve water security for the three billion people globally affected, but the future looks grim. For many communities, from Cape Town, South Africa to Flint, United States, the imagined dystopian future of severe water shortages has already arrived-shaped not so much by lack of water, but by aging infrastructure, underfunded utilities, social exclusion, politicized commodification, and environmental racism. Stepping off from my biocultural research in Cochabamba, Bolivia, I discuss how recent research is dramatically advancing our understanding of water insecurity, such as new findings around the biocultural causes and consequences of dehydration, contamination, and water stress. But, much more needs be done to support local communities in creating fair and just water systems. I discuss how human biologists can make crucial contributions toward the advancement of a much-needed science of water insecurity, while highlighting some practical and ethical challenges to advancing a core mission of providing safe, sufficient water to all.