In industrial countries, 1 child out of 500 develops a cancer before the age of 15 years, and before the age of 6 years for almost half of them. In France, incidence rates were stable over the 15 last years with around 1500 cases each year. A very small fraction of cases is attributable to known risk factors, including heritable cancers or cancers in children with heritable predisposing diseases, cancers induced by high doses of ionizing radiation of medical or accidental origin, by chemotherapeutic or immunosuppressive drugs. Responsibility of Epstein Barr virus in a fraction of Hodgkin's and Burkitt's lymphomas is also established, even though little is still known on the cofactors involved in industrial countries. Other virus could cause leukaemia, as suggested by localized increases in incidence in various conditions of population mixing. Conversely, there is some evidence that early common infections could be protective toward leukaemia risk, probably through their contribution to the maturation of the immune system. Several agents are suspected to induce chemical cancers, particularly pesticides, which are consistently reported in childhood leukaemia and brain tumours. It is more and more likely that genetic factors may modulate risk induced of environmental factors.