Wiskott-Aldrich syndrome (WAS) and X-linked thrombocy topenia (XLT) are caused by mutations of the WAS protein (WASP) gene. The manifestations of the classic WAS phenotype consist of immunodeficiency, eczema and thrombocytopenia. However, thrombocytopenia and small platelets are the only consistent features of WAS and XLT. The exact mechanisms of the development of thrombocytopenia in patients with WAS or XLT are unknown. To date, platelets are the only primary cells in which inducible tyrosine phosphorylation of WASP has been consistently demonstrated. This review focuses on the recent progress in dissecting the causes of thrombocytopenia and the emerging understanding of WASP phosphorylation.