The definition of concussion has changed many times over the last 50 years; however, in general, concussion is a subset of mild traumatic brain injury and represents a traumatically induced transient disturbance in neuronal function that cannot be otherwise explained. Though the exact pathophysiology of concussion is unknown, it is thought that during a concussion a force is delivered to the brain by any of multiple modalities (e.g., physical force such as head impact or pressure force such as blast injury) that creates a stretch of the neurons, which in turn disrupts neuronal functions and communication. This disruption also results in some extent of direct structural cell injury and impairs their ability to repair themselves, making them more vulnerable to a second and more harmful later concussion. Though most concussions occur due to a physical force impacting the head and translation of that force to the brain, blast concussions are common in those that serve in the military, and it is generally thought to occur when a nearby explosion changes the intra and extracranial pressures and causes an acceleration force to the head and brain, resulting in neuronal damage.