Intermittent exotropia (X(T)) is a large exophoria that intermittently breaks down to an exotropia. Occluding one eye breaks fusion and will manifest the exotropia (Fig. 5.1). When fusing, the eyes are straight and stereoacuity is excellent, 40 s of arc. When tropic, there is large hemiretinal suppression of the deviated eye. It is common for patients to show a preference for one eye, but the clinician should resist the temptation to label the deviation as a right or left exotropia, as the deviated eye can easily be changed by covering the dominant eye. Patients with late onset of exotropia during late childhood or adulthood may experience diplopia when tropic. The exotropia is typically manifest when the patient is fatigued, daydreaming, or ill. Approximately 80% of intermittent exotropia patients will show progressive loss of fusion control and an increase in the exotropia over several years. Adult patients can have extremely large deviations.