Deficient abduction can be due to both innervational deficiencies and/or mechanical restrictions. Innervational deficiencies include lateral rectus weakness or paralysis, myasthenia gravis, progressive external ophthalmoplegia nd Duane's syndrome with aberrant innervation to the lateral rectus muscle. Mechanical causes include restrictions on the medial side of the globe (a tight medical rectus due to endocrine ophthalmopathy, a contractured medial rectus muscle as a result of a lateral rectus paralysis, excessive resection of the medial rectus muscle, scarring and hemorrhage due to multiple previous operations upon the medial rectus), as well as a posteriorly inserted lateral rectus. This latter condition may result from a traumatically disinserted lateral rectus muscle, an excessively recessed lateral rectus muscle or a lateral rectus that has slipped from its attachment following strabismus surgery. Insufficient force for full abduction is caused by posterior insertion of the lateral rectus. The diagnosis of these various conditions is considered with reference to the forced duction, force generation and saccadic velocity tests. The treatment of these different causes of abduction deficiency is discussed noting that the approach to treatment depends upon the etiology of the abduction deficiency.