Strabismus is defined as an ocular misalignment. Since it can cause not only impaired visual function but also social handicap and tremendous emotional stress, the care of patients with strabismus should include psychological and social aspects. Although strabismus is one of the major fields in pediatric ophthalmology and neuro-ophthalmology, its precise mechanism and etiology are still unknown. It can be inherited from strabismic parents, or be derived from the anomalous structure, neurologic deficits, and refractive errors. The diagnosis of strabismus can be made by covering one eye, and the degree of strabismus can be quantified by the alternate prism cover test. Recently MRI is used widely for the diagnosis of various anomalous orbital and muscular structures, especially to investigate heterotopia of extraocular muscle pulley. The treatment modalities for strabismus are either surgical or nonsurgical. Surgical treatments can be made by recession or resection of the involved extraocular muscle. The adjustable suture technique was introduced in 1970s, which has been the gold standard among surgical treatment modalities. Nonsurgical treatments include prism, glasses, bifocal lenses, and drugs. A young strabismic patient may have amblyopia and decreased stereoacuity due to abnormal interaction between the sound eye and the deviating eye. Once amblyopia is detected, immediate treatment is needed to correct the visual dysfunction. Recent efforts to elucidate the mechanisms of strabismus are believed to unravel the mysterious pathophysiology in the near future.