The X-ray emission from galactic and extragalactic compact objects is dominated by continuum processes. However, spectral lines, both in emission and absorption, albeit rare, are very sensitive probes of the physical conditions in the vicinity of the central objects. Relativistically broadened iron lines, cyclotron lines, surface lines on neutron stars and accretion disk corona lines have been reported for a large variety of systems as the sensitivity and resolution of instrumentation has improved. Missions currently planned such as ASTRO-H and ATHENA will bring a considerable advancement to the field, although even lower-resolution instruments can contribute to specific aspects of line spectroscopy.