Erectile dysfunction usually has more than one cause, and this chapter explains the main ones. It begins with physical causes, particularly cardiovascular and metabolic risk factors, and explains how atherosclerosis can reduce blood flow to the penis as well as the heart. Neurological conditions such as Parkinson's disease and spinal cord disorders are covered, along with hormonal problems. The chapter then looks at psychological factors including stress, anxiety, depression and relationship difficulties, medications that can affect sexual function, and lifestyle habits like heavy drinking, smoking and illicit drug use.