The organ is one of the oldest instruments in Western music, and its sound has inspired many of the greatest composers from the past half a millennium from Bach through to Messiaen. The full possibilities of the instrument, however, have often been overlooked. Orchestration textbooks tend to mention the organ only in passing, with little practical advice for the composer to latch onto. Many organ books will explain technical jargon and how the organ is designed, but do not discuss what should actually be written in the score. This book, on the other hand, offers a practical guide for composers on how to write idiomatically for the instrument with the aim of helping them exploit the instrument's full timbral and technical potential. It provides in depth advice not only on notation but also the full resources of the modern organ, including dedicated chapters on registration, writing for manuals and pedals, and using the organ in ensembles, among others. Multiple musical examples are quoted from across the history of organ repertoire, with a particular focus on contemporary composers and music. There is advice tailored to non-classical musicians, such as guidance on virtual instruments. The online companion website to this book provides video demonstrations, chapter summaries, sample organ stop-lists and other useful further resources. In summary, the goal of this book is to show not just what organ music was, or what it is, but what it can be.