Infectious mononucleosis (IM) is the manifestation of primary infection with Epstein-Barr virus (EBV). EBV persisting after infection for a life-time infects > 90% of the adult population. Primary infection mostly asymptomatic in young children manifests in teenagers and young adults in about 50% as IM with fever, sore throat, generalized lymphadenopathy, frequently hepatosplenomegaly and blood lymphocytosis with the characteristic atypical lymphocytes. Clinical presentation, typical lymphocytosis and heterophile antibodies are diagnostic. Atypical cases may need to be confirmed by specific serology. IM is a self-limiting lymphoproliferation regressing within 2-3 weeks. Complications are rare and may involve many different organs. Severe cases are very uncommon, except in patients with inborn or acquired immunodeficiency carrying a substantially higher risk for severe courses, pogredient lymphoproliferation and lymphoma.