The definition of myocardial contractility remains complex and its exact measurement remains difficult, especially in the intact heart. Measurement of load-independent effects requires sophisticated equipment and preparations that are not applicable to clinical practice. At present, the more attractive concept is to think in terms of the calcium-contractile protein interaction, especially at the level of calcium-troponin-C. Either an increased supply of calcium is required to bind to the regulatory site on troponin-C, or modifications in troponin-C are required to promote interaction with the contractile system at the same level of cytosolic calcium. In other words, contractility can best be conceived of as the calcium-contractile protein interaction. There are, in addition, other sites at which calcium can act, such as by enhancing the myosin ATPase activity. Calcium-dependent phosphorylation of myosin light chains can also occur but is probably of much greater importance in vascular smooth muscle. Once calcium has interacted with troponin-C to promote the strong binding state, the crossbridge interaction has a cooperative effect whereby any attachment of crossbridges enhances binding of calcium to troponin-C. Hypothetically, even those troponin-C molecules not being activated by calcium can by this mechanism participate in the contractile process.