We have been privileged to start our academic careers at the begin ning of the decade in which the immunological roles and hypersensitivity diseases of the gastrointestinal tract and liver have been defined. In the early 1960s IgA was reported to be the main secretory immunoglobulin, immunoblasts were shown to home to the intestinal mucosa and certain serum autoantibodies were described in patients with chronic liver disease. Shortly thereafter IgE and Australia antigen were discovered. Parallel advances in clinical investigation, in particular closed biopsy techniques, facilitated correlation of morphological changes with im munological mechanisms in disease of the gastrointestinal tract and liver. Only 10 years later, the concepts of immunity and hypersensitivity are regularly applied to the pathogenesis, diagnosis, treatment and prog nosis of many chronic diseases in these organs. In designing this book we have attempted to integrate theorectical and clinical immunology as they pertain in 1975; our ultimate aim is aptly described by Brachet as quoted by Professor Paronetto (page 319). We would like to think that this review provides a basis for the next major advances in the fields of gastrointestinal and hepatic immunology. As we see it, the outstanding problem in both sites is how to produce protective immunity without hypersensitivity.