Erectile dysfunction (ED) is a highly prevalent condition among males all over the world. Earlier, psychogenic causes were supposed to be the most common cause, but recently organic causes such as hypertension, cardiovascular disease, dyslipidemia, diabetes, illicit drugs and depression are found to be the most common. It is more understood now and varieties of tests are available to diagnose and treat this condition. ED may form the basis of medicolegal investigation, both in civil and criminal cases. Penile anatomy, physiology of sexual intercourse, pathophysiology of ED, epidemiology, etiology, assessment of a case of ED, laboratory investigations, neurological assessment, medicolegal issues in relation to ED, etc. are also described in this chapter. 10 well-illustrated case studies related to the ED are given in this chapter.