This chapter highlights neurocognitive changes that occur in adults. Neurocognitive disorders affect the way the brain functions. Neurocognitive disorders include conditions such as mild neurocognitive disorder, delirium, and dementias (now known as major neurocognitive disorder in the DSM-5). Alzheimer disease, Parkinson disease, Huntington disease, and multiple sclerosis are well-known neurocognitive disorders. These disorders can have a genetic component and can be associated with environmental factors. They can also be caused by an underlying medical condition, such as a traumatic brain injury or stroke, metabolic conditions, and even infections. Continued research is important for discovering new treatments, medications, and preventive strategies for neurocognitive disorders.