This chapter provides an overview and synthesis of the cognitive impairments that are commonly associated with traumatic brain injury (TBI) in adults. It summarizes deficits in fundamental processes that support most domains of cognition like reaction time, attention and self-awareness and the major domains of cognition ranging from general intelligence, memory, and spatial cognition. The importance of cognitive diagnosis and treatment stems from the fact that rehabilitation outcomes and long-term effects of TBI on personal, social, and occupational functioning can often be related to the integrity of cognitive abilities along with behavioral and emotional adjustment. Neuropsychological assessment encompasses clinical and psychometric testing procedures that survey and objectively measure the effects of cerebral damage on cognition, behavior, and social-emotional functioning. A multidisciplinary and comprehensive approach to TBI and specifically to cognitive and behavioral impairments is necessary for best treatment outcomes.