These smallest cells of cellular component of blood carry great responsibility of hemostasis. Their role in thrombosis, hemorrhage and inflammatory processes resulting into MI, stroke, DIC, like life-threatening crisis, attracts the attention of practitioners belonging to all clinical disciplines. This chapter discusses various causes of abnormal function and numbers of platelets. This chapter highlights the importance of Mean platelet volume (MPV) which is a measurement that describes the average size of platelet in the blood which is an extremely valuable parameter, but often underutilized, and guides for the real need of platelet transfusion.