Despite being poorly understood and historically underrecognized (Beresin, J Geriatr Psychiatry Neurol 1:127–143, 1988), delirium is an acute cognitive disorder frequently encountered by inpatient providers. Delirium has important predisposing and precipitating risk factors, complex pathophysiology, and variable clinical course that can lead to negative long-term outcomes. For this reason, prompt diagnosis and treatment are essential, and neuropsychologists are uniquely positioned to identify and differentially diagnose delirium from other, similarly presenting conditions. This chapter will provide an overview of the symptoms, classification, and pathophysiology of delirium, offer guidance for assessment and intervention, and review literature pertaining to outcome and prognosis. Recommendations for practice approaches are designed to aid the reader in working with this important patient population.