Numerous causes for failure of the penis to achieve adequate rigidity for vaginal penetration have become apparent, and sophisticated methods have been developed for determining more accurately the particular cause in a given case. The therapeutic options remain limited, however, especially in patients with true organic disease. Fortunately, the surgically implantable penile prosthesis has evolved into an effective, reliable means of establishing penile rigidity suitable for intromission. This chapter will deal with such prostheses from the standpoint of historical significance, patient selection, device selection, surgical technique, and future considerations.