Satellite altimetry systems are designed to map the sea surface. These systems measure the satellite-to-sea surface round-trip travel time of radar or light pulses to determine the height of the satellite (altimetric range) above the instantaneous sea surface. The difference between the satellite altitude above a reference surface and the altimetric range provides the sea surface height with respect to the same reference surface. The range from the satellite to the sea surface is corrected for various components of the atmospheric refraction and to mitigate effects caused by instrumental biases and sea state induced systematics. A number of corrections due to different geophysical effects are also taken into account. Different products are distinguished: Satellite altimeters cover the ocean areas with repeated tracks at repetition rates of some days and permit the estimation of sea surface heights to an accuracy of 1 to 2 cm. The original altimetry data measured by the individual satellite altimetry missions are provided by the responsible space agencies. Some of them also provide pre-processed sea surface heights (so-called Level-3 or Level-4 products). Refined solutions are computed by different agencies, universities and research institutes, who provide their results in grids of few minutes of arc. These products differ with respect to the used data sets, the considered time span, and the processing strategies and methods. Satellite altimetry enables a nearly global and quasi-continuous monitoring of the sea level with cm-accuracy. The comparison of altimetry solutions referring to different epochs allows the estimation of sea surface and sea level variations, which provide insights in the redistribution of oceanic water masses, thermal expansion and contraction, ocean current variability and eddies, density variations due to water temperature and salinity, atmosphere-ocean coupling, as well as exchange with fresh waters, including melting ice sheets and glaciers.