The penile blood vessels arise from the internal pudendal artery. After the arcuate ligament the artery is usually called the main penile artery. Afterwards the vessel is divided into the dorsal penile artery, which is thin, and the cavernosal artery, which enter the corpus cavernosum on the supero-medial surface of the penis. The branches of this artery are called the arteriae helicinas that subsequently divide in smaller vessels that communicate with the lacunae of the corpus cavernosum. The venous blood flow is drained into the venous plexus beneath the tunica albuginea. The emissary veins perforate the albuginea and the blood is drained by the venae circumflexae into the deep dorsal vein. Color-Doppler sonography allows actually to depict all these vessels and to study the blood flow in these structures with the exclusion of the lacunar spaces. To detect slow flows the technical parameters are very important and the Doppler angle must be always less than 60 degrees. During pharmacologically induced erection the peak systolic velocity in the cavernosal artery shows different values, according to the time of measurements. Initially high systolic velocity is detected (over 35 cm/s) and high diastolic flow (8-15 cm/s) due to low intracavernosal resistance. After veno-occlusive mechanism activation, an increase of flow is normally observed with peak systolic velocity up to 80-120 cm/s associated to progressive decrease of diastolic flow. In the initial phase of the erection the venous flow is characterized with elevated blood velocity in the circumflex and deep dorsal veins. Afterwards with progressive erection the venous flow reduces to disappear completely with full rigidity. Three dimensional US allows a better visualization of the morphology, number and distribution of the vascular tree of the penis.