Erectile dysfunction is characterized by the regular or repeated inability to obtain or maintain an erection. There are several ways that erectile dysfunction is analyzed:

- Obtaining full erections at some times, such as when asleep (when the mind and psychological issues if any are less present), tends to suggest the physical structures are functionally working. However the opposite case, a lack of nocturnal erections, does not imply the opposite, since a significant proportion of sexually functional men do not routinely get nocturnal erections or wet dreams.

- Obtaining erections which are either not rigid or full (lazy erection), or are lost more rapidly than would be expected (often before or during penetration), can be a sign of a failure of the mechanism which keeps blood held in the penis, and may signify an underlying clinical condition, often cardiovascular in origin.

- Other factors leading to erectile dysfunction are diabetes mellitus (causing neuropathy) or hypogonadism (decreased testosterone levels due to disease affecting the testicles or the pituitary gland).

Erection problems are very common. The Sexual Dysfunction Association estimates that 1 in 10 men in the UK have recurring problems with their erections at some point in their life.

Penile erection is managed by two different mechanisms. The first one is the reflex erection, which is achieved by directly touching the penile shaft. The second is the psychogenic erection, which is achieved by erotic or emotional stimuli. The former uses the peripheral nerves and the lower parts of the spinal cord, whereas the latter uses the limbic system of the brain. In both conditions an intact neural system is required for a successful and complete erection. Stimulation of penile shaft by the nervous system leads to the secretion of nitric oxide (NO), which causes the relaxation of smooth muscles of corpora cavernosa (the main erectile tissue of penis), and subsequently penile erection. Additionally, adequate levels of testosterone (produced by the testes) and an intact pituitary gland are required for the development of a healthy male erectile system. As can be understood from the mechanisms of a normal erection, impotence may develop due to hormonal deficiency, disorders of the neural system, lack of adequate penile blood supply or psychological problems. Restriction of blood flow can arise from impaired endothelial function due to the usual causes associated with coronary artery disease, but can also include causation by prolonged exposure to bright light or chronic exposure to high noise levels.