Long-baseline interferometry has been traditionally regarded as a very technical method with a limited number of applications. At the turn of the millennium, with the introduction of large community-oriented facilities such as the VLTI, we are seeing a transformation into a major astronomical technique, with a corresponding dramatic increase in the number of publications. This book marks this transition, by providing a wide and deep insight into a large number of diverse and compelling results recently obtained by long-baseline interferometers. It also provides an insight into concepts which form the basis of future instruments and further developments in this technique.