Cardiac myocytes or cardiomyocytes are the most physically energetic cells in the body. They are highly specialized high-oxygen-content cells that house a large number of mitochondria. They occupy as much as 75% of the cardiac mass, but constitute only about one third of the total cell number in the heart. Differentiated cardiac myocytes have little capacity to proliferate; however, hypertrophic growth has been shown to respond to alpha1-adrenergic stimuli via the Ras/MEK pathway. All cardiac myocytes are capable of spontaneous rhythmic depolarization and repolarization of their membranes. Contraction of cardiac myocytes is myogenic, which is independent of nervous stimulation. There is a complex network of signals in cardiac myocytes regulating the rhythmic pumping of the heart. Cardiac myocyte hypertrophy and apoptosis have been implicated in the loss of contractile function during heart failure. A better understanding of the cardiac signaling network will help reveal the cellular mechanisms leading to cardiac myocyte death.