The International Association for the Study of Pain (IASP) defines pain as "...an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage".

Pain is part of the body's defense system, triggering a reflex reaction to retract from a painful stimulus, and helps adjust behaviour to increase avoidance of that particular harmful situation in the future. It is a major symptom in many medical conditions, significantly interfering with a person's quality of life and general functioning. Diagnosis is based on characterizing pain in various ways, according to duration, intensity, type (dull, burning or stabbing), source, or location in body. Usually pain stops without treatment or responds to simple measures such as resting or taking an analgesic, and it is then called 'acute' pain. But it may also become intractable and develop into a condition called chronic pain, in which pain is no longer considered a symptom but an illness by itself. The study of pain has in recent years attracted many different fields such as pharmacology, neurology and psychology.

Nociceptive pain and neuropathic pain are the two main kinds of pain when the primary mechanism of production is considered. Nociceptive pain may be classified in three types that have distinct organic origins and felt qualities:
• Superficial somatic pain (or cutaneous pain) is caused by injury to the skin or superficial tissues. Cutaneous nociceptors terminate just below the skin, and due to the high concentration of nerve endings, produce a sharp, well-defined, localized pain of short duration. Examples of injuries that produce cutaneous pain include minor wounds, and minor (first degree) burns.
• Deep somatic pain originates from ligaments, tendons, bones, blood vessels, fasciae, and muscles. The scarcity of pain receptors in these areas produces a dull, aching, poorly-localized pain of longer duration than cutaneous pain; examples include sprains, broken bones, and myofascial pain.
• Visceral pain originates from body's viscera, or organs. Visceral nociceptors are located within body organs and internal cavities. The even greater scarcity of nociceptors in these areas produces pain that is usually more aching or cramping and of a longer duration than somatic pain. Visceral pain may be well-localized, but often it is extremely difficult to localize, and several injuries to visceral tissue exhibit "referred" pain, where the sensation is localized to an area completely unrelated to the site of injury. • Neuropathic pain is damage to the nervous system itself, due to disease or trauma, may cause neuropathic (or neurogenic) pain. Neuropathic pain may refer to peripheral neuropathic pain, which is caused by damage to nerves, or to central pain, which is caused by damage to the brain, brainstem, or spinal cord.

Medical management of pain has given rise to a distinction between acute pain and chronic pain. Acute pain is 'normal' pain, it is felt when hurting a toe, breaking a bone, having a toothache, or walking after an extensive surgical operation. Chronic pain is a 'pain illness', it is felt day after day, month after month, and seems impossible to heal. In general, physicians are more comfortable treating acute pain, which usually is caused by soft tissue damage, infection and/or inflammation among other causes. It is usually treated simultaneously with pharmaceuticals, commonly analgesics, or appropriate techniques for removing the cause and for controlling the pain sensation. The failure to treat acute pain properly may lead to chronic pain in some cases. General physicians have only elementary training in chronic pain management. Often, patients suffering from it are referred to various medical specialists. Though usually caused by an injury, an operation, or an obvious illness, chronic pain may as well have no apparent cause, or may be caused by a developing illness or imbalance. This disorder can trigger multiple psychological problems that confound both patient and health care providers, leading to various differential diagnoses and to patient's feelings of helplessness and hopelessness. Multidisciplinary pain clinics are growing in number since a few decades.