The Alpha-Particle X-ray Spectrometer (APXS) is part of the scientific payload of all four Mars rovers to date. It determines the chemical composition of rocks and soils using X-ray spectroscopy during irradiation with alpha particles and X-rays from 244 cm. All elements heavier than fluorine can be detected by their characteristic X-ray lines. Typically, 16 elements are quantified for each martian sample. An additional 10 trace elements can be quantified for unusual high abundances. The APXS has provided compositional data at 4 landing sites, analyzing more than 1000 samples along a combined traverse of ~70 km. The diverse composition of soils and rocks has provided insights about martian geology and environmental conditions. Soils at all landing sites are similar and basaltic, but enriched in S, Cl, and Zn, likely from volcanic exhalations. A variety of igneous rocks have been documented. High sulfur concentrations in Ca sulfate veins, ferric sulfate subsurface soil deposits, and the extensive Burns formation with ~30% sulfate indicate extensive interactions with acidic fluids in the past. APXS bulk geochemistry complements mineralogy data and images and delivers crucial constraints for the interpretation of other investigations, like ground truth for orbital remote sensing instruments or comparison with martian meteorites.