At present, beam ion beam therapy has started to spread worldwide. In Europe and Asia, combined carbon/proton facilities are favored, but in the US, only proton centers are under construction. This development is partially due to the different funding procedures and partially due to the more complex physical and especially biologic features of the heavy ions. In this article, the basic properties of both ions are presented, and their features for therapy are outlined. This refers to the dose conformity, the general precision of the treatment, and the ability to monitor via in-beam positron emission tomography the ions range inside the patient. Then the very complex biologic features are treated, and, finally, the treatment plans are compared.