Improving knowledge about the mechanisms of radiation-induced tumor development is the goal of the DISCOVER Project, funded by the EU with more than 1.3 million euros and coordinated by ENEA in collaboration with the Federal Office for Radiation Protection (Germany), National Centre for Public Health and Pharmacy (Hungary) and Oxford Brookes University (Britain). Ionizing radiation is electromagnetic or corpuscular radiation endowed with sufficient energy to "ionize" the matter it passes through, in practice to set in motion large numbers of electrically charged particles. The U.S. Academy of Science estimates that 82% of the radiation to which each individual is exposed on average comes from natural sources, such as Radon gas. The remaining 18%, resulting from human activities, is mainly from medical procedures using X-rays, and only to a lesser extent from the use of consumer products, nuclear medicine, occupational reasons, or nuclear fallout/spills. Radiation tumor formation is classically attributed to unrepaired or mispaired DNA damage. However, recent data suggest a contribution of the cellular microenvironment, challenging conventional (target theory) radiobiology.