There are 16 operating proton therapy centers world-wide in 1994. By July 1994, proton beams had been used to treat ∼ 14000 patients for both benign and malignant disease. Six centers are located in Europe, four in the United States, three in Russia, two in Japan and one in South Africa. Accelerators originally designed for other purposes and adapted for radiation therapy are employed at all but one center which has an accelerator designed and built for the hospital environment. At six centers the accelerator has a maximum energy of < 100 MeV; only a limited number of sites can be treated. Proton beams of variable energy are available at six centers, otherwise additional material is placed in the beam line to degrade the energy. Passive scattering techniques are used to spread the beam in the lateral dimension. Ridge filters or rotating propellers of various materials are used to modulate the beam with depth.