In France, cancer hits around 1,700 children (0-14 years) and 700 adolescents (15-19 years) each year. In other terms, one child out of 440 developes a cancer before the age of 15 in industrial countries, and 1 out of 300 before the age of 20. Five-year survival after childhood cancer has dramatically improved in the last 30 years, reaching yet 80%. A very small fraction of cases is attributable to known risk factors such as heritable cancers and heritable predisposing diseases, high-dose ionizing radiation, chemotherapeutic drugs. Other factors are suspected, such as lack of common infections in early childhood in leukaemia, or prenatal exposure to household pesticides in many cancers. To cite this journal: Oncologie 13 (2011).