The last quarter of the 20th century witnessed the rebirth and maturing of optical interferometry and associated technologies. Major successes spanning from direct detection of stellar pulsations to imaging in the optical were achieved with test-bed systems, some of which have now evolved to facilities open to the astronomical community. The intense activity and rapid growth of this field are a clear sign that interferometry will be a major observational tool in this century both from ground and space. This book presents state of the art optical interferometry in astrophysics. It emphasises new VLTI users by including tutorials in optical interferometry theory and practice, and related instrumentation, as well as reviews in stellar formation and evolution, and extragalactic science.