This chapter presents the history, clinical findings, prevalence, etiology, and management of nightmares in children and adult. Nightmares are REM sleep phenomena and often occur in the second half of the night. Upon awakening, the person is oriented about her/his surroundings and can give a detailed description of the dream action. Several researchers conceptualize nightmare as a primary sleep disorder, because empirical evidence clearly demonstrates low subjective sleep quality in nightmare sufferers, presumably due to direct effects and indirect effects. Nightmare frequency is higher in girls and women, and decreases based on age. The prevalence of children experiencing nightmares once a week or more often is estimated to be about 5%. The etiology of nightmares is best described by a disposition-stress model. The chapter presents case reports of a child and an adult suffering from nigntmare disorders.