Platelet transfusion recipients become alloimmunized to foreign HLA and HPA antigens, impairing their responses to further platelet transfusions (platelet transfusion refractoriness; PTR). Anti-HLA and HPA antibodies account for the majority of clinically significant alloimmunizations. Poor transfusion responses due to alloimmunization may be overcome by selection of HLA or HPA-compatible donors. Several approaches including use of white cell-depleted products have been shown to prevent or delay alloimmunization in transfusion recipients. This paper focused on: 1) factors involved in the development of platelet alloimmunization, 2) diagnosis of PTR and platelet alloimmunization, 3) effective management of the alloimmunization.