This chapter discusses the physiology of haemostasis, which is a collective term for mechanisms to prevent blood loss. Platelets, also known as thrombocytes are small and irregularly shaped blood cells. Platelets participate in almost all stages of haemostasis. The characteristic features of platelets are an extensive canalicular system and a microtubular system composed of contractile proteins. Platelets are formed in the bone marrow. Platelets are the smallest blood cell but have the largest precursor cell. Megakaryocytes develop demarcation membranes within their cytoplasm and then form pseudopodia like processes. Thrombopoiesis seems to be regulated by a group of humoral factors collectively called thrombopoietins. Contractile proteins of the platelets bring about clot retraction. Vasoconstriction tends to stop bleeding. Collection of platelets at the site of injury also leads to aggregation of platelets because of their stickiness. Coagulation is the end result of a series of chemical reactions involving plasma proteins.