The Epstein-Barr Virus (EBV) is the causative agent of Infectious Mononucleosis (IM). IM is an acute illness characterized clinically by sore throat, fever and lymphadenopathy. In children, 5-10% of primary EBV infections are asymptomatic. By adulthood, 90 95% of most populations have demonstrable EBV antibodies. EBV specific serodiagnostic tests are not usually needed in typical cases of IM, since 90% of the cases are heterophile-positive. For heterophile-negative cases and for diagnosis in atypical cases, confirmation of infection by EBV-specific serology is useful. The serodiagnosis of IM is established with a single acute phase serum by the detection of elevated levels of IgM and IgG antibodies to VCA. Antibody conversion or decline in VCA-IgM antibodies (they usually disappear within 3-6 weeks after onset) and the emergence of EBNA antibodies (they arise from 3 weeks to several months after onset) can confirm diagnosis. Previous EBV infections are identified by IgG antibodies to VCA and to EBNA.