The cellular events underlying Epstein–Barr (EB) virus infection in man are considered in respect of three situations. Firstly, the events of the Primary infection leading to infectious mononucleosis are described and illustrate the multiplicity of virus-cell interactions and of immnological responses involved. Secondly, possible mechanisms to explain EB virus persistence in the virus-immune host are discussed in the light of new studies monitoring the various indices of the virus carrier state. Thirdly, the analysis of EB virus infection in immunosuppressed patients shows how a relaxation of T-cellmediated immunity alters the position of the virus-host balance.