Thrombin is a Na⁺-activated, allosteric serine protease that plays multiple functional roles in blood pathophysiology. Binding of Na⁺ is the major driving force behind the procoagulant, prothrombotic and signaling functions of the enzyme. This review summarizes our current understanding of the molecular basis of thrombin allostery with special emphasis on the kinetic aspects of Na⁺ activation. The molecular mechanism of thrombin allostery is a remarkable example of long-range communication that offers a paradigm for many other biological systems.