Sulfur is a widely distributed element on Earth and in the solar system. Its multiple valence states (S2− to S6+) allow it to participate in numerous geochemical and biochemical processes. It may be one of the light elements in the Earth's core and may have been crucial in core formation. Sulfur is an essential component in all life on Earth and likely supported earliest life. Sulfur geochemistry is used to understand the early evolution of Earth's atmosphere and hydrosphere, and serves as a monitor of volcanic SO2 and H2S and as a tracer of anthropogenic sources of sulfur. Recent advances in the use of multiple sulfur isotopes (32S, 33S, 34S, and 36S) and in situ isotopic measurements will help to develop sulfur stable isotopes as a vital tracer in the Earth and planetary sciences and will provide applications for understanding inorganic and biogenic processes.