This chapter describes the two calcium channels, Dihydropyridine receptors and Ryanodine receptors that cause increase in intracellular calcium as the action potential arrives, leading to Excitation-contraction coupling at the T tubules flanked by sarcoplasmic reticulum cisternae. It also discusses the role played by cellular calcium ions during ventricular systole and diastole, the energy consuming mechanisms of cardiac cell and the molecular events leading to pressure development in left ventricle.