Post-transplant lymphoproliferative diseases (PTLDs) are a clinically and morphologically heterogeneous group of lymphoid proliferations. They represent a life-threatening complication of solid organ transplantation. The mechanisms of their pathogenesis are not yet fully understood. A combination of impaired immunity, oncogenic consequences of immunosuppressive therapy and EBV infection may play a role. Studies on incidence, treatment and prognosis are difficult because of the small number of cases occurring at each transplant center and the lack of reliable classification. Overall mortality remains high even though 25% of patients require no other measure than reduction in immunosuppression which must be the first step of treatment. Several treatments are currently used but more adequate classification as well as multicenter studies are urgently needed because many questions remain with regard to therapeutic strategy. Late-onset monoclonal tumors may be treated by conventional chemotherapy, while EBV-positive PTLDs may benefit from other approaches such as antiviral therapies or immunologic modulation of tumor functions.