The study of disease entities as complex as Crohn's dis­ ease will increasingly require comprehensive knowledge of formerly unrelated areas of the medical sciences. To promote this broad approach, a conference was organized in which geneticists, morphologists, immunologists, and virologists participated as well as clinicians whose work is focused on Crohn's disease. Ample time was given to the presentation of major new findings in each of these areas, and comments were given by the participants in the various sections. This approach yielded many new ideas, because indiv iduals with very different backgrounds were able to address old problems from fresh angles. This volume, published in January, 1981, contains the papers presented dur ing the wor kshop held in June, 1980, in Noordwijk/Leiden, The Netherlands. In addition, we have in­ cluded the extensive discussions, edited by experts in the field, which followed each presentation. Finally, each main category is