Molecular typing for platelet allelic polymorphisms was first made possible by discovery of the HPA-1a/1b single nucleotide polymorphism in 1989. Since then, six other biallelic human platelet antigen (HPA) systems have been determined and can be typed using genomic DNA. The introduction of polymerase chain reaction enabled development of several different assays including polymerase chain reaction-sequence-specific primer, melting curve analysis by LightCycler, and 5'-nuclease assays. More recently, multiplex polymerase chain reaction has allowed for the development of high-throughput assays for genotyping large numbers of patients and blood donors for not only platelet gene polymorphisms but also for those of other blood cell genes. Platelet genotyping is a valuable tool in confirming platelet antigen specificities of alloantibodies detected in patient sera to complement the clinical history in the diagnosis of alloimmune platelet disorders such as fetal and neonatal alloimmune thrombocytopenia (FNAIT), posttransfusion purpura, and multiplatelet transfusion refractoriness. In addition, it has made possible prenatal platelet typing of the fetus in suspected cases of FNAIT and large-scale blood donor typing for provision of antigen-negative platelets to transfuse highly alloimmunized patients. Platelet genotyping may also someday prove important as an aid in determining the relative risk of patients for various thrombotic disorders.