The development of human semilunar valves was studied in a range of specimens including embryos before the appearance of cellular semilunar valves at stage 17 and hearts after the attainment of mature fibroelastic valvular structure, which occurs around the time of birth. The valves develop by modification of endocardial cushion material at the downstream end of the cardiac tube and appear to grow at the margins, probably by cellular proliferation into a stagnant zone caused by boundary layer separation of blood flow. The flat endothelium on the ventricular aspect and the cuboidal endothelium on the arterial aspect of the valves correlate, respectively, with expected high and low shear forces produced by surface blood flow. Development of an aorta and pulmonary trunk with tricuspid semilunar valves appears to be contingent on the appearance of separate entwined ventricular ejection streams. The later fibroelastic phases of semilunar valve development show progressive increase in elastic and collagenous fibers, at sites which appear to be subjected, respectively, to fluctuating and static tensions.