The Gravity Recovery and Climate Experiment Follow-on (GRACE-FO) mission is a partnership between NASA and the German Research Centre for Geosciences (GFZ). Scheduled for launch in May 2018, the twin GRACE-FO satellites will pick up where the GRACE mission (2002-2017) left off, following the evolution of our planet’s ever-changing water cycle by measuring and monitoring month-to-month changes in how mass is redistributed within and among Earth’s atmosphere, oceans, land and ice sheets, as well as within Earth itself. It will do this by making very precise monthly measurements of changes in the shape of Earth’s gravity field. These data provide unique insights into Earth’s changing climate, Earth system processes and even the impacts of some human activities, and have far-reaching benefits to society, from chronicling the ongoing loss of mass in the Greenland and Antarctic ice sheets to helping water managers better measure the impact of droughts and forecast floods. The mission will also demonstrate a new technology that offers the potential to improve the already remarkable precision of its measurement system by a factor of 10 or more on future generations of GRACE-like missions.