Hydrogeodesy is the discipline that measures the Earth's solid and aquatic surfaces and gravity field from space to study water resources and their changes over time. The Special Collections on Hydrogeodesy: Understanding Changes in Water Resources Using Space Geodetic Observations, in Water Resources Research (WRR) and Geophysical Research Letters, was envisioned as a community initiative to: (a) understand the state-of-the-art of the approaches, concepts, and tools of hydrogeodesy, (b) explore hydrogeodetic innovations that improve the understanding of the water cycle and water resources, and (c) assess the potential of hydrogeodesy to address complex water-related and sustainability questions. The Special Collection includes 41 published articles studying various types of water resources and hydrological properties using the main four hydrogeodetic technologies: Altimetry, Interferometric Synthetic Aperture Radar, Gravimetry, and Global Navigation Satellite Systems. The collection is also timely in highlighting the novel hydrogeodetic advances and opportunities on the 60th anniversary of WRR, and showcasing the potential of the field to solve water-related and sustainability challenges.