The release of numerous PMP upon platelet activation is an efficient means for platelets to maximize their PPL surface for anchoring and assembling procoagulant or anticoagulant factors, thereby accelerating hemostasis locally at sites of activation. It is evident from this review that PMP almost certainly play additional roles in normal and pathological hemostasis. They participate in thrombus formation, fibrinolysis, leukocyte adhesion, platelet-endothelium interaction, and probably other activities, e.g. modulating phospholipase (see Section 3.5). This review covers the history of PMP, assay methodologies, factors influencing their production, their relevance in blood banking, but emphasizes studies of PMP in clinical disorders. A major purpose of this review is to bring these topics to wider attention in the hope that a consensus will emerge. Perhaps the most general controversy to be settled by future research is the extent to which PMP are causative agents in pathology, or are mere epiphenomena, i.e. incidental consequences.