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. The VLTI is the largest ground-based interferometric facility combining four 8.2-m telescopes with up to eight 1.8-m telescopes. This facility is the first opened on a shared risk basis in 2002, a milestone for the astronomical community. The combination of enhanced sensitivity and common user support bring into grasp a vastly unexplored astrophysical territory. This book presents state of the art optical interferometry in astrophysics. We emphasise 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.