Hadrontherapy was born in 1938, when neutron beams were used in cancer therapy, but it has become an accepted therapeutical modality only in the last fifteen years. Fast neutrons are still in use, even if their limitations are now apparent. Charged hadron beams are more favourable, since the largest specific energy deposition occurs at the end of their range in matter. The most used hadrons are at present protons and carbon ions, which allow a dose deposition which conforms to the tumour target.

Radiobiological experiments and the results of the first clinical trials indicate that carbon ions have, besides this macroscopic property, a different way of interacting with cell at the microscopic level. There are thus solid hopes to use carbon beams of about 4500 MeV to control tumours which are radioresistant both to X-rays and to protons.

After discussing these macroscopic and microscopic properties and presenting the work carried out at CERN in the framework of the Proton Ion Medical Machine Study (PIMMS), the hospital-based facilities in the world, running or under construction, will be reviewed.