Publisher Summary This chapter describes the myriad of factors that influence an individual's report of neurocognitive impairment, provides information related to definitions and categories of cognitive impairment, and reviews assessment procedures used in diagnosing cognitive disorders. Motor vehicle collisions (MVC) involvement may result in a variety of distress reactions, the type and intensity of these distress reactions determined by parameters of the collision; the number and severity of physical injuries and impairments; the level of pain that accompanies injuries; and the level of compromise to various lifestyle domains and overall quality of life. Cognitive impairment can be transient, temporary, fulminating, or permanent. Some people involved in MVC can have long-term neurological, psychiatric, physical, or psychological problems. These problems, singly or in combination, can have an adverse impact on thinking skills. The complexity lies in accurately identifying problems with cognition, quantifying the deficits, estimating the impact on day-to-day functioning, and apportioning causation. All of these factors, singly or in combination, can have an adverse impact on cognition. Following an MVC, the most common causes of cognitive impairment are traumatic brain injury, depression, posttraumatic stress disorder, chronic pain, and chronic sleep disturbance.