Transplantation of solid organs and hematopoietic stem cells is accompanied by profound disturbance of immune function mediated by immunosuppressive drugs or delayed immune reconstitution. Disturbed T cell control of Epstein-Barr virus (EBV)-infected B cells leads to posttransplant lymphoproliferative disorder (PTLD) in up to 10% of patients. Children are at a higher risk because they are more often EBV-naïve before transplantation. Patients with PTLD often present with unspecific symptoms (pain and organ/graft dysfunction). Depending on the onset of PTLD, manifestations vary between mainly nodal (late PTLD) and extranodal sites (early PTLD). Histology, immunohistology, EBER in situ hybridization and molecular pathology are required for diagnosis and subclassification of PTLD. The three major types are early lesions (resembling reactive proliferations in immunocompetent patients), polymorphic PTLD (proliferation of B and T cells with effacement of histoarchitecture) and monomorphic PTLD (presenting as malignant lymphomas, mainly high-grade B cell lymphomas). In a subfraction of cases, including monomorphic PTLD, reduction of immunosuppressive medication alone is sufficient to induce remission. Surgical debulking of tumor mass and anti-CD20-antibody treatment with or without chemotherapy (usually at lower dosages than in immunocompetent patients) constitute the basis of additional therapy.