Physical, technical, radiological, and clinical status of proton therapy is reviewed. Protons produce effects similar to those of x rays, but dose distribution and range make protons more flexible and useful therapeutically. The ability to confine the major fraction of proton absorbed dose to a designated volume allows the decrease of dose to normal tissue or the increase of dose to the cancer. Improved dose distribution is quantified by determining the ratio of normal tissue dose for x rays to that for protons in different treatment plans. Normal tissue integral dose from x-ray therapy is generally 2 to 5 times as high as that anticipated from proton therapy.