Over two years of GRACE time-variable gravity data provide a new means for monitoring water mass changes within the Earth system. Work to date has demonstrated a close correspondence between GRACE terrestrial water storage change estimates and observations/models, and has provided new insight into the magnitude and pathways of water mass redistribution among Earth's land, ocean, atmosphere and ice components. Here we review work to date, with an emphasis on the hydroclimatology of the continents, continental-scale mass changes, and implications for land contributions to sea level rise. GRACE-derived water storage changes will be compared to model output and observations for large river basins and for the global land surface. Implications and new applications for Earth system monitoring will be discussed.