Particle beams from protons to carbon ions have a number of important advantages for radiation therapy. Besides the physical selectivity that makes ions especially efficient for deep-seated tumours and tumours close to critical organs, carbon ions with their enhanced relative biological effectiveness offer an additional biological advantage for slow-growing radioresistant tumours, with an additional gain for tumours with hypoxic sections. These advantages can only be fully exploited by active beam-shaping and biology-based treatment planning.