Proton beams have been used for medical applications for nearly 40 years. Now, in 1989, more than 8000 patients have been treated worldwide. Techniques have been developed to allow uniform dose distributions to be delivered to a treatment volume which is tailored precisely to an extended target volume. Using these techniques in conjunction with the superior imaging methods now available allow the optimal dose to be delivered to the target while sparing adjacent normal tissue. The present worldwide experience, using the accelerators originally designed for nuclear physics research, is leading to increasing interest in having proton accelerators with more sophisticated beam delivery systems designed and built for use in a hospital setting.