Dreams occur during all stages of sleep. Nightmares are common. They can be associated with poor sleep and diminished daytime performance. Frequent nightmares are not related to underlying psychopathology in most children and in some "creative" adults. However, recurrent nightmares are the most defining symptom of post-traumatic stress disorder and may be associated with other psychiatric illnesses. Night terrors are arousal disorders that occur most often in children and usually occur early in the sleep period. Patients with rapid-eye-movement behavior disorder often present with nocturnal injury resulting from the acting out of dreams. Dream disorders may respond to medication, but behavioral treatment approaches have shown excellent results, particularly in patients with post-traumatic stress disorder and recurrent nightmares.