Optical stellar interferometry has entered a new growth era and its use has improved the capabilities well beyond those of traditional telescopes. This is the first book to be written on the subject. Building on a basic knowledge of physical optics, the authors describe the ideas behind interferometric techniques in optical astronomy, how they are put into practice, and how they can be used in the future. They then introduce basic optics, to establish a physical and mathematical framework to discuss the ideas and instruments in interferometry, including an overview of operational and planned interferometric observatories. Finally, they discuss important astrophysical discoveries made through interferometry, and present as-yet untested ideas which will enable us to image details of planetary systems beyond our own. This book will be used by advanced students in physics, optics, and astronomy who are interested in the ideas and implementations of astronomical interferometry.