Erectile dysfunction (ED) is the inability to achieve or maintain an erection sufficient for satisfactory sexual performance. It is common, affecting at least 12 million U.S. men. The five-question International Index of Erectile Function allows rapid clinical assessment of ED. The condition can be caused by vascular, neurologic, psychological, and hormonal factors. Common conditions related to ED include diabetes mellitus, hypertension, hyperlipidemia, obesity, testosterone deficiency, and prostate cancer treatment. Performance anxiety and relationship issues are common psychological causes. Medications and substance use can cause or exacerbate ED; antidepressants and tobacco use are the most common. ED is associated with an increased risk of cardiovascular disease, particularly in men with metabolic syndrome. Tobacco cessation, regular exercise, weight loss, and improved control of diabetes, hypertension, and hyperlipidemia are recommended initial lifestyle interventions. Oral phosphodiesterase-5 inhibitors are the firstline treatments for ED. Second-line treatments include alprostadil and vacuum devices. Surgically implanted penile prostheses are an option when other treatments have been ineffective. Counseling is recommended for men with psychogenic ED.