Primary and secondary brain injury are ways to classify the injury processes that occur in brain injury. In traumatic brain injury (TBI), primary injury occurs during the initial insult, and results from displacement of the physical structures of the brain. On the other hand, secondary injury occurs gradually and may involve an array of cellular processes. Secondary injury, which is not caused by mechanical damage, can result from the primary injury or be independent of it. The fact that people sometimes deteriorate after brain injury was originally taken to mean that secondary injury was occurring. It is not well understood how much of a contribution primary and secondary injuries respectively have to the clinical manifestations of TBI.