Tennis elbow, also known as lateral epicondylitis or enthesopathy of the extensor carpi radialis origin, is an enthesopathy (attachment point disease) of the origin of the extensor carpi radialis brevis on the lateral epicondyle. The outer part of the elbow becomes painful and tender. The pain may also extend into the back of the forearm. Onset of symptoms is generally gradual although they can seem sudden and be misinterpreted as an injury. Golfer's elbow is a similar condition that affects the inside of the elbow.

Enthesopathies are idiopathic, meaning science has not yet determined the cause. Enthesopathies are most common in middle age (ages 35 to 60).

It is often stated that the condition is caused by excessive use of the muscles of the back of the forearm, but this is not supported by experimental evidence and is a common misinterpretation or unhelpful thought about symptoms. It may be associated with work or sports, classically racquet sports, but most people with the condition are not exposed to these activities. The diagnosis is based on the symptoms and examination. Medical imaging is not particularly useful. Signs consistent with the diagnosis include pain when a subject tries to bend back the wrist when the wrist is against resistance.

The natural history of untreated enthesopathy is resolution over a period of 1-2 years. Palliative (symptoms alleviating) treatment may include pain medications such as NSAIDS or acetaminophen (paracetamol), a wrist brace or strap over the upper forearm. The role of corticosteroid injections is debated. Recent evidence suggests corticosteroid injections may delay symptom resolution.