Fetal and neonatal thrombocytopenia can be induced by a maternal alloimmunization against fetal platelet antigens. Alloimmune thrombocytopenia occurs with an incidence of 1/1,000 livebirths and may present either with evidence of damage from a prenatal hemorrhage such as porencephaly or intrauterine death, or with active life-threatening bleeding during the neonatal period. It is due to the destruction of fetal platelets by alloantibodies reacting on specific antigenic sites. In Caucasians, the human platelet antigen 1a (HPA-1a) is the most frequently involved in alloimmune thrombocytopenias, accounting for 80-90% of the cases. Anti-HPA-5b is responsible for a further 5-15% of the cases. This article reviews the clinical aspects, the biological diagnosis and the management, including prenatal sampling and maternal therapy.