During the last decade, a variety of morphological, physiological, and biochemical studies have provided an improved definition of the alterations in myocardial structure and function that herald the sequential stages of cardiac growth and development. InitiallY, there is a commitment of mesodermally derived progenitor cells to the cardiac muscle cell lineage that is followed by the formation of a primordial heart tube. Organogenesis proceeds through a series of involutions of the heart tube and the onset of septation, which results in distinct cardiac chambers and the acquisition of regional-specific properties of atrial, ventricular, and conduction system cells. The subsequent enlarge ment of the embryonic heart is largely dependent on an increase in myocyte number, which continues until shortly after birth, when cardiac myocytes lose their proliferative capacity and acquire the terminally differentiated phenotype of adult cardiac muscle cells. In response to hormonal, physiological, hemodynamic, and pathological stimuli, adult ventricular muscle cells can adapt to increased workloads through the activation of a hypertrophic process, characterized by an increase in the contractile protein content of individual