This review was completed in January 1973 Contents 1. Introduction and history . . 321 2. Microwave cavity, power generation and linear accelerator principles . . 323 2.1. Introduction . . 323 2.2. Microwave cavities . . 324 2.3. Microwave power generation . . 325 2.4. Travelling wave accelerating structures , . 327 2.5. Standing wave accelerating structures . . 328 3. R'adiotherapy treatment units , . 331 3.1. Introduction . . 331 3.2. Isocentric mounting . * 333 3.3. A4uxiliary systems: vacuum, beam-transport, electronics . 334 3.4. Operating parameters and performance . . 337 3.5. Radiation monitoring and safety features . . 339 3.6. Chronological review of treatment units . . 343 4. Contemporary developments . . 344 4.1. Computer control, monitoring and record keeping . . 344 4.2. Cryogenic accelerators with superconducting cavities . . 346 4.3. Microtrons . . 347 24 M W peak and 20 kW average power output wit'h an efficiency of 40%.