Fever is a common symptom in children and may sometimes be prolonged or recurrent. There are many differential diagnoses, which may lead to significant diagnostic delay. Diagnosis is based on the clinical presentation as well as a widespread panel of investigations that are necessary in order to exclude the many potential causes of fever before reaching a definite diagnosis. In particular, the physician will look for infections and malignancies before considering the disease as inflammatory. This chapter reviews the differential diagnosis of prolonged or recurrent fever, and discusses most of the inflammatory syndromes presenting with fever.