In recent years, accelerated heavy charged‐particle (proton and light‐ion) beams have been clinically used at an increasing number of accelerator facilities worldwide. Several hospital‐based accelerator facilities dedicated to radiation therapy of cancer have been constructed, and their number is growing. Descriptions are presented of diverse instruments that have been developed in modifying extracted particle beams for clinical application, measuring the delivery of treatment beams, and controlling the treatment process to ensure patient safety. ©1995 American Institute of Physics.