The relationship between sleep deprivation, sleep disorders, and neurocognitive impairments is bidirectional. Sleep deprivation in neurologically normal individuals may cause cognitive and psychiatric impairments. Primary sleep disorders may also mediate sleep fragmentation that causes neuropsychiatric symptoms or aggravates comorbid neuropsychiatric conditions. This chapter reviews the importance of normal sleep to normal neurocognitive functioning and performance, considers the impact of acute and chronic sleep deprivation on cognitive faculties in neurologically normal individuals on vigilance, psychomotor performance, and mood, then reviews the current understanding of cognitive and psychiatric symptoms and problems characteristically resulting from, or aggravated by, common sleep disorders, focusing especially on cognitive impairments associated with sleep-disordered breathing. A practical clinical approach towards assessment of cognitive impairment and psychiatric illness in sleep medicine practice is then outlined, focusing on bedside evaluation of the patient and important diagnostic and therapeutic implications. Recognition of the frequency and morbidity of cognitive and psychiatric symptoms caused by sleep deprivation and sleep disorders is growing, and comprehensive care in sleep medicine requires that practitioners should consider and address neurocognitive and psychiatric manifestations in their patients.