The sonographic appearance of the penis varies according to the degree of rigidity. In the flaccid state, the corpora cavernosa appear as two, dorsally located, circular structures with homogeneous mixed echotexture and intermediate echogenicity as a result of the innumerable interfaces created by their complex system of vascular sinusoids. The corpus spongiosum appears as a ventrally located circular structure with homogeneous echotexture and higher echogenicity with respect to the corpora cavernosa (Figs. 45.5, 45.6, 45.7, and 45.8) [11]. The glans is more echogenic than the corpora cavernosa [5]. The normal urethra is visualized as a tubular anechoic structure with a thin, smooth echogenic wall, and the lumen distends to at least 4 mm in diameter [12]. When collapsed, the urethra appears as a transverse line. The tissue layers surrounding the corpora are only partially identifiable in the flaccid state [15]. Skin, subcutaneous tissue, and dartos cannot always be readily separated [15]. An extremely thin hyperechoic line identifies the interface formed by the deep fascia of the penis [15]. The Colles fascia is barely visible [5, 16]. The tunica albuginea and the Buck’s fascia are usually stuck together and appear as a thin (usually less than 2 mm) echogenic layer surrounding the corpora [5, 16]. The two distinct layers become appreciable only when fluid extravasation accumulates between them or very high-frequency transducers are used [5]. In some cases, vascular structures may provide a suitable interface to separate small portions of the Buck’s fascia from the underlying tunica albuginea in normal conditions as well. In particular, the Buck’s fascia becomes visible at ultrasound near dilated circumflex veins, and a subtle echogenic line representing the Buck’s fascia is usually recognized in the dorsal aspect of the penis dividing the plane of the deep vessels from that of the superficial vessels [5]. The thickness of the tunica albuginea in the flaccid state is about 1–3 mm [16]. The echoes from the tunica albuginea are specular reflections and thus are demonstrated with efficiency only when the ultrasound beam is perpendicular to them. Perpendicular insonation, in particular, is of paramount importance to evaluate the echogenicity of the plaques [9]. The penile septum appears as an echogenic structure with back attenuation dividing the corpora cavernosa that can hamper visualization of the tunica albuginea in the dorsal aspect of the penis [5]. The distal penile ligament is recognized at ultrasound as a linear structure more echogenic than the surrounding glanular tissue located centrally within the glans dorsal to the distal urethra [5]. Several penile vessels can be identified at gray-scale ultrasound. The cavernosal arteries appear as a pair of dots located slightly medially in each corpus cavernosum. On longitudinal scans, they present as narrow tubular structures with echogenic wall (Fig. 45.9) [15, 17]. The diameter of the normal cavernosal arteries ranges from 0.3 to 05 mm in the flaccid state. The dorsal arteries are visible in the dorsal aspect of the shaft as anechoic structures with a similar diameter to the cavernosal arteries [5]. Dorsal veins present with less echogenic wall compared to the arteries [5]. When the penis becomes erect, the two corpora cavernosa enlarge and the sinusoids dilate [1, 15]. The echogenicity of the corpora cavernosa progressively decreases during tumescence starting from the region surrounding the cavernosal arteries because of sinusoids dilatation [5]. During maximal penile rigidity, a fine echogenic network is appreciable in the corpora cavernosa due to sinusoidal interfaces (Fig. 45.10) [5]. Sinusoidal spaces at the base of the penis are normally larger with respect to the remaining portions of the shaft. Blood entrapped within the sinusoids often appears slightly corpusculated [5]. During erection, the fibrous layers that envelop the corpora can be best visualized. The tunica albuginea thins and its thickness is about 0.5 mm [16]. The intracavernous pillars are recognizable on transverse scans as straight echogenic lines thicker than the sinusoidal walls, which run from one side to the other of the tunica albuginea [5]. The diameter of the normal cavernosal arteries increases to 0.6–1.0 mm after an intracavernosal injection of vasoactive agents. Moreover, during the onset of erection, cavernosal artery pulsation is evident in normal subjects [5, 18]. As occurs for the cavernosal arteries, also the diameter of the dorsal arteries increases during erection but to a lesser extent compared with the cavernosal arteries [5].