Low-energy ion-scattering spectroscopy (LEISS) is used extensively to analyze solid surfaces. The LEISS process relies on binary elastic collisions between an incident ion beam and the atoms in a sample to obtain information on the surface atoms. The velocity of the scattered ions is used to determine the mass of the atoms that are struck. This article introduces LEISS and its principles. It describes the use of LEISS spectra in qualitative and quantitative analyses, and reviews the instrumentation and applications of LEISS.