Penile erection is a complex phenomenon that includes coordinated interaction of the nervous, arterial, venous, and sinusoidal systems. A defect in any of these systems may result in erectile dysfunction. Erectile dysfunction is defined as consistent inability to generate or maintain an erection of sufficient rigidity for sexual intercourse. Doppler US is a first-line imaging modality to detect vascular causes of erectile dysfunction. The parameters that indicate the presence of arterial disease are a subnormal clinical response to vasoactive agents, a less than 60% increase in the diameter of the cavernosal artery, and a PSV of the cavernosal arteries less than 30 cm/s. If a significant discrepancy exists between the velocities of two cavernosal arteries (>10 cm/s difference), unilateral arterial disease should be suspected. In the presence of normal arterial function, Doppler findings suggestive of abnormal venous leak are persistent EDV of the cavernosal artery greater than 5 cm/s and demonstration of flow in the deep dorsal vein. The development of reversal of the diastolic flow in the cavernosal arteries after intracavernosal injection of vasoactive agents is regarded as a reliable indicator of venous competence. Other conditions of the penis that may cause erectile dysfunction and may be evaluated with US and Doppler US are Peyronie’s disease, priapism trauma-related abnormalities, and penile neoplasms.