The cognitive system is comprised of a hierarchy of basic and complex cognitive processes. Depending on the specific task demands, different levels of processes are recruited for successful task completion. Brain injury causes a disruption of neuronal systems that may interfere with the efficiency of the cognitive hierarchies. This chapter presents the neuropathological mechanisms that affect basic and complex cognitive systems, current theories for recovery of function, and the effects of brain injury on attention, learning, and memory; categorization abilities; and executive functioning (EF). It integrates principles of modern cognitive neuroscience in an innovative manner for the treatment of attention and categorization deficits associated with traumatic brain injury and concludes the current state of the art on efficacy research.