Discusses the evidence that informs current etiologies of fever, stratified by regional geography, by presenting the clinical presentation, diagnosis, and treatment of the most common diseases, with special considerations for (1) certain age groups, (2) the burden of disease for different conditions, (3) classification and treatment strategies, and (4) a review of available diagnostic tests, as well as different health systems approaches to diagnosis and treatment of the febrile child at the community and health-facility levels, the evidence base for WHO-sponsored approaches such as Integrated Management of Childhood Illness (IMCI), and Integrated Community Case Management (iCCM). Fever remains one of the most common presenting symptoms of pediatric illnesses, with fever in children under age five years signifying systemic inflammation, typically in response to a viral, bacterial, parasitic, or less commonly, a noninfectious etiology. Patients’ ages and geographic settings can help direct the appropriate diagnostic approach and treatment, if local epidemiology is well understood