Preclinical experimental transplantation research that is based on microsurgical models in rats fulfills two indispensable conditions for modern organ transplanta­ tion research: Almost all organ grafts can be performed on the rat with an amount of technical effort that is still justifiable. Thus transplantation models that are analogous to human organ transplantation can be developed, tested, and evaluated. This fulfills a necessary condition from the standpoint of surgery. With the species rat, we have a great variety of genetically different inbred strains. From the immunological point of view this is an indispensable prerequisite for the investigation of preclinical transplantation models that can be expected to produce controllable, reproducible results. In vivo experimental results can be supplemented by and correlated to in vitro tests. Lately these experimental results are being greatly expanded and more precisely defined by the application of immunohistological methods that have been established recently in Kiel. In this book we hope to present a cross section of the microsurgical models in use today and of current immunological and immunohistological models. Furthermore, we wish to record the present state of microsurgical organ transplantation research and to show its relationship to the current state and development of clinical organ transplantation. A special aspect of our Kiel research group is the long-term, well-functioning, interdisciplinary cooperation between surgery, immunology, and pathology. Through this cooperation we attempt to provide an atmosphere in which theoretical and practical viewpoints can mutually influence each other.