Posttransplant lymphoproliferative disorders (PTLD) represent an immunosuppression-related lymphoid or plasmacytic proliferation that occur in the setting of solid organ transplant or allogeneic hematopoietic stem cell transplantation (HSCT). PTLD is a devastating consequence of HSCT and solid organ transplantation with a high morbidity and mortality. Most commonly, PTLD is related to Epstein–Barr virus (EBV) infection, but an increasing number of non–EBV-related cases are occurring. Initial therapy involves withdrawal of immunosuppression with or without antibody or cytotoxic chemotherapy. Novel therapeutic approaches including EBV-specific cytotoxic T lymphocytes are currently being studied.