This article reviews the essential primary and secondary injuries attributable to traumatic brain injury (TBI) which causes one third of all injury deaths in the United States. Motor vehicle crashes, falls, assaults, guns, sports, and recreational activities are the major causes of TBI. Secondary peak incidences of TBI occur in infants and children and the elderly. Conditions that increase risk for accidents include alcoholism, prior head injury, prior meningitis, seizure disorders, mental retardation, and psychiatric disorders. However, gunshot wounds to the head are steadily increasing and since 1990 have caused more deaths each year than motor vehicle accidents. The incidence, severity, etiology, and specific types of injuries have been assessed in clinicopathologic studies of head injuries. The pathologic features of both the primary and secondary lesions attributed to TBI should be understood by anyone caring for head-injured patients. The computed tomography (CT) and magnetic resonance (MR) images mirror the pathologic abnormalities found in head trauma. Radiologists must accurately interpret the CT and MR images of injured patients. Forensic pathologists have long appreciated the characteristic focal lesions, such as coup and contracoup contusions, that occur in falls or vehicle accidents, but the understanding of diffuse injuries has been more elusive. Understanding the nature of the focal and diffuse injuries is critical to understanding the morbidity and mortality of brain injury.