Haemostasis is a word of Greek etymology, αἱμόστασις, literally meaning the stoppage of blood (αἱμό = blood, στασις = stasis = standing still). The term haemostasis designates the process that prevents blood from flowing outside blood vessels in case of vascular wall injury. The haemostatic system is complex and includes many actors, from both the blood vessel wall and the circulating blood itself. Multiple interactions between these numerous and interdependent actors allow a state of homeostasis, in which a balance is maintained between haemorrhagic and thrombotic tendencies in a healthy individual. The primary goal of clot formation is to provide an immediate defence against bleeding at sites of vascular wall injury before repair mechanisms re-establish vascular wall integrity. Associated regulatory mechanisms prevent excessive clot extension and allow clot dissolution once the acute phenomenon is under control. These physiological adaptive mechanisms can however be dysfunctional or altered in various medical conditions, leading to haemorrhagic or thrombotic complications. The aim of this chapter is to describe the basic principles of normal haemostasis and clot formation. Then, the main mechanisms underlying pathological arterial and venous thrombosis will be presented, highlighting their differences and their common features.