Serine protease inhibition by proteins of the serpin family is a unique and complex process involving physical, chemical, and conformational changes. After encounter with the reactive site of inhibitor, the protease is conformationally trapped as a covalent complex resembling the acyl-protease intermediate of catalysis. The stability of the trap is not permanent and may vary for different proteases. In addition, the trapping mechanism is not 100% efficient and a fraction of the serpin may be consumed like a substrate before inactivation is complete. Characterization of protease-serpin inhibition therefore requires the measurement of three parameters: the apparent second order rate constant of inhibition (k(inh)), the stoichiometry of inhibition (SI), and the rate of complex breakdown (k(brkdn)). The basic kinetic methods to establish these parameters are described.