Graft-versus-host Disease (GvHD) occurs primarily but not exclusively as a complication in the context of allogeneic bone marrow or peripheral blood stem cell transplantation (BMT or PBSCT), the treatment of choice for various life-threatening diseases. GvHD may affect skin, liver, gut, and other organs and sometimes runs a devastating or even lethal course. The diagnosis of GvHD is based on both clinical and histomorphological parameters. However, due to the widespread use of GvHD prophylaxis in patients undergoing allogeneic BMT or PBSCT, the number of histologically typical lesions has decreased, while the number of atypical, low-grade or masked lesions has increased. Therefore, an update of the clinical and diagnostic pathology of GvHD of skin, liver, gastrointestinal tract and other organs is required. In the present volume the histological features of GvHD lesions under today's conditions are described and illustrated in detail. Particular emphasis is placed on differential diagnostic problems and histodiagnostic pitfalls. Finally, the applicability and limits of immunohistological methods for the diagnosis of GvHD are shown.