In the human body, an intricate mechanism has evolved to limit blood loss from damaged blood vessels through formation of clot, while maintaining blood in a fluid state, such that the circulation remains intact. This system is required for maintaining the integrity of the circulatory system as perturbations in the balance between procoagulant and anticoagulant forces can lead to bleeding or thrombotic disorders. Formation of the key coagulation enzyme, the serine protease thrombin, proceeds through a tightly regulated series of reactions involving a cascade of activation of plasma proteases and cofactors. The steps in the serine protease activation cascade are regulated by inhibitors, termed serpins, also called serine protease inhibitors. In order to define serpin functions, it is also necessary to understand their target proteases. One of the central protease pathways is the thrombotic or clot-forming pathway. The tissue factor and contact activation pathways both activate the final common pathway of factor X, thrombin, and fibrin. In this chapter we define the basic clinical methods used to measure factor X (FX).