The detection of platelet antigens and platelet antibodies has always been difficult. Recent technical achievements are due to the availability of more specific and sensitive reagents (i.e. F(ab)2 fragments; higher specific activity of labels; monoclonal antibodies directed against platelet membrane constituents etc.) and the application of advanced immunological assays to platelet immunology (i.e. blotting assays, “capture”-ELISA's and radioimmunoprecipitation in conjunction with SDS polyacrylamide gel electrophoresis). These techniques have permitted the definition and immunochemical characterization of new platelet allo- and autoantigens, have assisted in clinical diagnosis and promoted our understanding of pathogenic mechanisms. Illustrative examples are presented and discussed.