Harmful algal blooms (HABs) and hypoxia (low-oxygen conditions) in the marine waters and freshwaters of the United States negatively impact resources across thousands of miles of the Nation’s coastal and inland waters. Their prevalence and duration encompass scientifically complex and economically damaging aquatic issues. This book assesses threats posed by HABs and hypoxia and provides a comprehensive research plan and action strategy outlining Federal agencies’ roles and responsibilities for evaluating and managing them.