The Epstein-Barr Virus (EBV), also known as Human Herpes Virus 4, has been identified and described as the etiological agent of infectious mononucleosis. It is the first human virus found to be implicated in oncogenesis. EBV is associated with a number of biologically diverse cancers, mainly lymphoproliferations of B-cell origin. With the development of basic research and clinical medicine, new ideas related to the pathogenesis and clinical aspects of EBV-associated diseases have arisen recently. A significant progress in the number of hematopoietic stem cell and solid organ transplantations performed worldwide, involving high-risk patients, has contributed to an increase in the development of EBV-associated malignant post-transplant lymphoproliferative disorders. In the stem cell transplant setting, a post-transplant lymphoproliferative disorder has in most cases the nature of a secondary cancer. Apart from post-transplant lymphoproliferative disorders, other types of lymphoproliferative disorders are recognized and presented in this book.