Core Messages: Posttransplant lymphoproliferative disorders (PTLDs) straddle the disciplines of immunology, infection, and malignancy. The study of PTLD could lead to advances that have ramifications beyond transplantation. Early observations revealed that lightening of immunosuppression could lead to regression in some cases, eliminating the hopelessness of the word lymphoma. As newer and more potent immunosuppressive agents were introduced, the frequency of PTLD increased, responses were often suboptimal, and the need for additional strategies for prevention and treatment became apparent. PTLD is now recognized as a major complication of transplantation, such that we now consider its study a discipline by itself.