This chapter describes the epidemiology, clinical diagnosis, and clinical significance of Fetal and neonatal alloimmune thrombocytopenia (FNAIT) in pregnancy. FNAIT is the commonest cause of severe neonatal thrombocytopenia, and is analogous to the fetal/neonatal anemia caused by hemolytic disease of the fetus and newborn (HDFN). FNAIT is usually suspected in neonates with bleeding or severe, unexplained, and/or isolated postnatal thrombocytopenia. The strategies for ante-natal treatment have included the use of serial platelet transfusions, which while effective are invasive and associated with significant morbidity and mortality. Significant recent progress has involved refinement of maternal treatment, stratifying it according to the likely severity of FNAIT based on the history in previous pregnancies. However, the ideal ante-natal treatment, which is effective without causing significant side-effects to the mother or fetus, has yet to be determined, and further clinical trials are needed.