The book covers all aspects – historical, theoretical, experimental – of quantum optomechanics and nanomechanics, with its applications to quantum measurement, foundations of quantum mechanics and quantum information. It is an essential read for any new researcher in the field. The summer school “Quantum optomechanics and nanomechanics,‘ held at the Les Houches School of Physics as its 105th session, was designed to summarize the progress made in the field, lay out the open questions, and define directions for future work. This book collects the lecture notes of the main courses given in this summer school.