Abnormalities in brain development, thought to be irreversible in adults, have long been assumed to underlie the neurological and psychiatric symptoms associated with neurodevelopmental disorders. Surprisingly, a number of recent animal model studies of neurodevelopmental disorders demonstrate that reversing the underlying molecular deficits can result in substantial improvements in function even if treatments are started in adulthood. These findings mark a paradigmatic change in the way we understand and envision treating neurodevelopmental disorders.