This chapter provides some basics of the interpretation of satellite altimetry measurements and explains how to use altimetry for hydrology. Satellite altimetry has been extensively used over the last two decades to calculate the water height variations over the Earth's lakes, rivers, reservoirs, and floodplains. Time series of altimetry-based water levels can be used to detect the signature of climate variability and extreme climatic events on the surface water storage. Echoes are acquired due to a tracking system placed on board the satellite known as "trackers". In the nominal tracking mode, the Jason-2 Poseidon-3 altimeter uses the "median tracker" concept, which again ensures the independence of shape pulse power weighting similar to the Free Model Tracker design on board Envisat. A dry tropospheric correction must be applied to the ranges in order to account for the delay of propagation of the radar pulse through the atmosphere.