Sea level rise is both a powerful impact of and indicator for global warming and climate change. Observing sea level change, as well as its causes, is therefore a top priority for scientists and society at large. By measuring the ocean’s temperature, salinity, mass, and surface height, the relative sources of recent sea level rise can be discerned. With these observations, sea level change is determined in terms of total sea level and its two major components, ocean mass and steric (density-related) sea level. The sea level budget is closed when the sum of the independent components agrees with measurements of total sea level, indicating that the observations can be used to interpret the causes of sea level change. While nearly global monitoring of sea level from space-based radar altimeters has been available since the early 1990s, satellite gravity missions capable of weighing changes in ocean mass have been available for less than 10 years. Even more recently, the Argo array of profiling floats achieved a level of coverage that now allows assessment of global sea level change due to temperature and salinity in the upper 2000 m of the ocean. Only during the overlapping period of all three observing systems can the sea level budget be directly addressed by observations.