Strabismus is a common group of disorders that have a significant genetic component. Within this highly heterogeneous group sits a small number of recognisable Mendelian conditions including certain congenital cranial dysinnervation disorders (CCDD) and mitochondrial cytopathies (that cause chronic external ophthalmoplegia).CCDDs are congenital, non-progressive conditions that result from abnormal development of the cranial nerves and their nuclei. They include both sporadic and familial disorders, and since they frequently involve the extraocular and facial musculature and result in abnormal eye movements, they often present to the ophthalmologist.CCDDs can be due to errors in cranial motor neuron identity or abnormalities of cranial nerve axon growth/guidance; these can result in phenotypes such as congenital fibrosis of the extraocular muscles or Duane syndrome. Notably, the phenotypic spectrum of CCDDs may include facial weakness (Moebius syndrome) or a range of neurological, cardiovascular and/or skeletal abnormalities (horizontal gaze palsy and scoliosis).