Hereditary defects in platelet function are responsible for sometimes severe mucocutaneous hemorrhages. They are a heterogeneous group of abnormalities whose first-line diagnosis typically involves interpreting the results of in vitro light transmission aggregometry (LTA) traces. Interpretation of LTA is challenging. LTA is usually performed in specialized laboratories with expertise in platelet pathophysiology. This review updates knowledge on LTA, describing the various platelet aggregation profiles typical of hereditary platelet disorders to guide the physician in the diagnosis of functional platelet disorders.