This chapter reviews the causes, diagnostic criteria, pathophysiology, sequelae, natural history, and factors affecting recovery and prognosis of mild and moderate traumatic brain injury (mTBI and ModTBI, respectively). It provides an overview of relevant research on mTBI and ModTBI as a resource for practitioners working within the medicolegal context. Incidence rates and causes are discussed across various demographic groups. The variable and nonspecific nature of mTBI sequelae is examined, including sex differences in symptom presentation, while the more severe and less ambiguous sequelae of ModTBI are also addressed. A thorough overview of factors contributing to TBI severity classification and the most recent diagnostic criteria for mTBI are discussed. The pathophysiology of mTBI is detailed, with a distinction between primary and secondary injuries in the context of ModTBI. The current state of the literature on biomarkers for diagnosis and prognosis of mTBI is also presented. Rates, correlates, and the degree of clinical recovery are described separately for mild and moderate TBI. Overall, this chapter aims to assist clinicians with the assessment and differential diagnosis of patients with mild and moderate TBI in a medicolegal context.