Crossed-eyes (strabismus) is caused when weak eye muscles don't work well together to align the eyes properly. This results in one or both eyes turning in, up, out or down instead of being aligned straight ahead. The condition usually develops by the time a child is 21 months old but may not become apparent until age six. People tend to believe the condition will be outgrown but this is not true. Neglecting crossed eyes will only make the condition worse. The condition may be treated using glasses, prisms or surgery in combination with vision therapy. In vision therapy the patient is taught to help both eyes work together which solves the underlying problem. Surgery is rarely prescribed alone because if the underlying problem is not corrected crossed eyes may return. Like most conditions affecting the eye early diagnosis and treatment is important.