This paper reviews the literature and describes our experiences in the diagnosis and treatment of tendinitis in horses. Ultrasonography provides a sensitive tool to diagnose tendinitis and quantitate the degree of damage to the tendon; as well as provide differential diagnoses such as peritendinitis. The principles in therapy of acute tendinitis are: Immediate reduced exercise or rest, physical therapy to reduce inflammation and administration of local and systemic antiinflammatory drugs. The goal is restoration of the tensile strength of the tendon without peritendinous granulation tissue and adhesions. To achieve this goal it is important to maximize the intrinsic healing and to minimize the extrinsic healing. Any form of counterirritation is forbidden because of increase of inflammation. Passive motion and massage will help to increase blood flow and to decrease adhesions. Local injection (peritendinous and intratendinous) of hyaluronic acid seems to increase the intrinsic healing and to decrease adhesions without side effects. Depending on the extent of the tendon damage, which tendon is involved and the progress toward healing controlled ultrasonographically, the healing period can be divided in 4 phases: 1. Immobilisation (Cast), only in severe cases (for 1-2 weeks). 2. Passive motion and massage (starting as soon as possible, usually at once during bandage change). 3. Careful exercise, hand walk or walk under saddle (for 1-6 months, starting as soon as possible, usually about 5 days after initial treatment). 4. Controlled slow and gradual training, no turnout, long or pasture (for 1-6 months, after phase 3). Chronic tendinitis is often caused when the severity of the initial injury is underestimated and the treatment inappropriate. Chronic tendinitis is treated using similar principles starting with phase 2. Tendon splitting and other surgical approaches have been used in selected cases to enhance the prognosis.