HIV infection in children is mainly the result of a mother-to-child transmission. The contamination during pregnancy is well known but intrapartum vertical transmission may also occur through ascending infection, blood exchange between mother and child, or direct contact with vaginal or cervical secretions. In addition HIV can be transmitted via breast milk. The reported rates of vertical transmission are highly variable: 14.4% in a European study, 18.3% in a French survey, 20 to 25% in the USA, 35 to 50% in Africa. It is unclear whether such a large variation of the rate of transmission is due to methodological differences or to different distributions of risk factors in the populations. There are some known predictive factors of HIV transmission such as low CD4 cells count, positive p24 antigenaemia and elevated concentrations of virus. The role of other factors is still debated: prematurity, virus (CMV, HTLV-1, HVB, HVC), C section prior labour, rupture of membranes. The prevention of HIV infection in infants is mainly based on contra-indication of pregnancy in infected women, desinfection of the vagina at the beginning of labour, early protection of the newborn by avoiding skin lesions and immediate washing, preventive treatment by zidovudin during pregnancy.