Bernard-Soulier Syndrome (BSS) is an autosomal recessive bleeding disorder due to quantitative or qualitative abnormalities in the glycoprotein (GP) Ib/IX/V complex, the platelet receptor for von Willebrand factor. BSS is characterized by giant platelets, thrombocytopenia, and prolonged bleeding time, and the hallmark of this disorder is the absence of ristocetin-induced platelet agglutination. In the last 10 years, the molecular and genetic bases of many GPIb/IX/V defects have been elucidated, providing a better understanding of primary hemostasis and structure-function relations of the complex. Thus far, more than 30 mutations of the GPIbalpha, GPIbbeta, or GPIX genes have been described in BSS. Recent studies also have shown that the phenotypes caused by mutations in the subunits of the GPIb/IX/V span a wide spectrum, from the normal phenotype, to isolated giant platelet disorders/macrothrombocytopenia, to full-blown BSS and platelet-type von Willebrand disease. Although recent progress in molecular biology has clarified the genotype-phenotype relationships of the GPIb/IX/V disorders, a close examination of platelet morphology on blood smears is still indispensable for a proper diagnosis. In this review, we summarize recent advances in the molecular basis of BSS with special emphasis on giant platelets and the genetic characteristics of Japanese BSS.