Idiopathic thrombocytopenic purpura (ITP) remains a clinical diagnosis made by the exclusion of other causes of thrombocytopenia. It is based on the patient's history, physical examination, and complete blood cell count, as well as examination of the blood film. Over the last four decades, a number of platelet antibody tests have been developed to aid the diagnosis of ITP. They can be divided chronologically into three groups. Phase I assays measure a functional change in control platelets after incubation with test serum. Because their sensitivity and specificity are low, they are no longer used to diagnose ITP. Phase II assays measure platelet-associated IgG by three different approaches. They lack the ability to differentiate between pathologic and nonpathologic platelet-associated IgGs. These assays are sensitive (80% to 90%) but their specificity is too low for them to be diagnostically useful. Phase III assays are the latest development in platelet serology testing. They measure glycoprotein-specific platelet antibodies by different approaches, namely, immunoblot, immunoprecipitation, and glycoprotein immobilization. Despite their high specificity, they suffer from low sensitivity (47% to 60%), which must be improved if they are to be clinically useful for the diagnosis of ITP.