Capillary density has historically been an important issue, beginning with the discovery of the circulation and extending to the present day. Determination of capillary density provided key evidence supporting the concept that the entire cardiac output passed through microscopic channels in the tissue and returned to the heart. Subsequently, control of capillary density by contractility of the capillaries themselves was proposed by Krogh. Later investigators disputed the notion of capillary contractility and proposed that changes in capillary density were secondary to changes in arteriolar tone or to passive mechanical factors. While this is still the prevailing view, the concept that the capillaries are not totally passive and may, through shape changes of the endothelia, alter the distribution of flow through the capillary network is under active consideration. While views of underlying mechanism vary, there is agreement that capillary density and its control are of fundamental importance to our understanding of the function of the microcirculation.