Charged-particle accelerators are the basic tools of high-energy physics, the science of fundamental properties of matter. Besides, a large number of synchrotron radiation sources are used world-wide for research and industrial purposes. Effective operation of modern accelerator facilities is almost impossible without precise and reliable diagnostics of particle beams. A review of instruments and methods of particle beam diagnostics in accelerators is given in this book. Physical principles of operation of beam diagnostic instruments of various types, such as contact probes, synchrotron radiation monitors and electromagnetic pickups are described. Methods of signal analysis are also reviewed. Bases of design and operation of beam diagnostic instruments are surveyed using particular examples. Methods of processing of the data obtained from beam diagnostic systems are analyzed in detail. Principles of a feedback usage in the particle accelerators are given. The book is intended for scientists and engineers occupied with the development and operation of particle accelerators, and can also be useful for the students training in particle beam physics and accelerator technique.