Epstein-Barr virus (EBV) is a ubiquitous virus that infects about 90–95% of the adult population. EBV establishes life-long latent persistence. The virus is found to be the major cause of infectious mononucleosis but it has also been associated with development of endemic Burkitt’s lymphoma. Result of EBV infection is the most common complication in patients after transplantation which is a post-transplant lymphoproliferative disease. Strong associations between EBV infection and Hodgkin’s lymphoma, nasopharyngeal carcinoma, gastric carcinoma and carcinomas derived from smooth muscle tissue also exist. There is a hypothesis that there is an association between EBV infection and autoimmune and allergic diseases. EBV is a Herpesvirus family member; its genetic material has dsDNA form. There are two strains of EBV: A and B. The only host for EBV is human with target cells: B cells and epithelial cells. The life cycle of EBV consists of lytic and latent phases. In the latent phase three different patterns of gene expression are possible. Due to some circumstances EBV can undergo reactivation, which is an important issue in transplantology. The main methods of diagnosis of EBV infections are serological methods that detect certain specific antibodies and recently more popular molecular biological methods such as PCR or in situ hybridization.