X-ray absorption spectroscopy (XAS) is widely used in earth sciences and in this chapter an overview is given of various sample systems, experimental approaches and scientific questions. XAS is used to characterize the structural environment of elements in various minerals or glasses of natural rock samples, to determine elemental oxidation states, to understand the incorporation of trace elements in minerals and to distinguish different phases. Further, XAS is used in laboratory experiments simulating the conditions of the inaccessible Earth in order to characterize the structural, chemical and electronic properties of crystalline and liquid materials down to conditions of the Earth's core–mantle boundary. In particular, X-ray absorption near-edge structure (XANES) data have provided unprecedented insights into redox equilibria in silicate glasses and melts, whereas the local structure of major and minor components is widely determined by extended X-ray absoption fine structure (EXAFS). Finally, XAS has substantially contributed to the understanding of element dissolution and transport by aqueous fluids in the Earth's crust.