A new heavy ion accelerator facility for radiotherapy is being designed at the Lawrence Berkeley Laboratory. Performance requirements have been established. Ions from helium to argon can be accelerated to a maximum energy of 800 MeV/nucleon with intensities in the range 108-109 particles per second. The accelerator subsystems consist of a linac injector, a synchrotron and a beam delivery system. Specifications have been developed for many of the technical components, and some details of the technical design are presented.