Cardiac function is determined by the coordinated and dynamic interaction of several cell types together with components of the extracellular matrix (ECM). This interaction is regulated by mechanical, chemical, and electrical signals between the cellular and noncellular components of the heart. Recent studies using fluorescence-activated cell sorting indicate that the number of myocytes remains relatively constant during development and disease, whereas the number of fibroblasts and other cell types can change dramatically. Cardiac fibroblasts appear to have different origins at different stages of development and fluctuate in response to a variety of physiological signals. Fibroblasts form a network of cells that are connected to each other via specific cadherins and connexins, to the ECM via integrins, and to myocytes by a variety of receptors, including connexins. Examples of the integration of signals include the role of angiotensin II (Ang II), which stimulates mechanical contraction of fibroblasts, as well as cytokine signaling. Cytokine signaling alters connexin and K(+) channel activation, which in turn is regulated by Ang II, essentially forming a feedback loop. Quantitative changes in mechanical, chemical, and electrical signals that can alter the overall cardiac form and function will be discussed here.