Human penile erection is a complex phenomenon mediated through the coordinated interaction among the nervous, vascular, and musculosystems. For social and ethical reasons, the mechanism and hemodynamics of this essential human function remained speculative and controversial for many years. In the past decade, however, advances in technology, together with a marked increase in clinical and research activity, have resolved some of the controversy and have provided a better understanding of erectile physiology. In this essay, we will review traditional concepts, as well as the new and exciting findings from many different research centers, concerning the physiologic mechanisms of penile erection that involves the arterial, venous, and nervous systems.