Clinical and Laboratory Standards Institute document H58-A--Platelet Function Testing by Aggregometry; Approved Guideline provides concrete, standard procedures for using aggregometry to assess platelet function in patient specimens and samples, with the intent to achieve greater uniformity of results by laboratories following these guidelines. Descriptions of light transmission aggregometry, whole blood impedance aggregometry, and shear-flow technologies are provided so both long-time and new users may establish consistent, reproducible platelet function testing programs in their laboratories.