The book is divided into two parts: Part I deals with the relevant physics and planning algorithms of protons, while Part II covers the radiobiology, radiopathology, and clinical outcomes of proton therapy, along with a comparison of proton therapy versus photon therapy. Protons can be used for radiosurgery and general radiotherapy. Since proton therapy was first proposed in 1946, about sixteen facilities have been built globally. Only a very few of these have isocentric beam delivery systems, so proton therapy is only now in a position to be compared directly through randomized clinical trials with modern photon radiotherapy systems, both for radiosurgery and for general fractionated radiotherapy. Three-dimensional proton planning computer systems with image fusion capabilities, including computerized tomography (CT) and magnetic resonance registration, are also discussed.