Adult-onset esotropiaEsotropia is commonly encountered in the context of lateral rectus weakness or thyroid eye diseaseThyroid eye disease, however there are numerous causes unrelated to either cranial nerve palsies or systemic pathology. These conditions are diverse but may be associated with myopic refractive errors or age-related tissue changes. This chapter will consider three significant causes of adult-onset esotropiaEsotropia: myopic esotropiaMyopic esotropia, muscle heterotopyMuscle heterotopy (heavy eyeHeavy eye syndrome) and age-relatedAge related esotropia esotropiaEsotropia (sagging eyeSagging eye syndrome). In each case the typical patient population, pathogenesis and management strategy will be discussed. A high index of suspicion should be maintained when assessing a new patient with adult-onset esotropiaEsotropia in order to ensure that more sinister underlying pathology is not missed.