Blood coagulation, also known as hemostasis, is the process by which blood changes from a liquid to a solid gel-like substance. It involves three stages: vasoconstriction, formation of a platelet plug, and coagulation of blood. When a blood vessel is injured, a series of reactions are initiated through the intrinsic and extrinsic pathways, ultimately resulting in a cross-linked fibrin mesh that traps blood cells to form a clot. Coagulation is tightly regulated by several mechanisms to prevent excessive clotting. Common tests evaluate the intrinsic and extrinsic pathways, as well as platelet count and function. Disorders can be inherited or acquired.