Platelets play a critical role in both primary and secondary haemostasis. In primary haemostasis, specialised glycoprotein receptors enable platelets to adhere to proteins that are exposed in areas of vascular damage. The process of adhesion, and/or the interaction of soluble agonists with receptors on the platelet, activates the platelets, which are then able to aggregate together. This aggregation creates a platelet plug that seals the breach in the vessel wall and prevents excess blood loss. Activated platelets then facilitate secondary haemostasis, the formation of a fibrin clot, by carrying coagulation factors and providing a catalytic surface for the major interactions of the coagulation cascade. This review briefly describes the adhesion, activation and aggregation of platelets; their role in blood coagulation and clot retraction; and how they may be inhibited in order to prevent thrombosis in at-risk patients.