Immunochemistry has traditionally focused on understanding the molecular basis of antibody binding affinity and specificity. In the last 20 years a major shift has occurred in the field. We are now asking how we can exploit the remarkable properties of the antibody molecule and the immune response in chemistry-from testing basic theories of enzymatic catalysis to the synthesis of new catalysts. Moreover, the concept of molecular diversity, which underlies these experiments, is now being applied to a large number of other problems-from libraries of biopolymers to libraries of solid-state materials. Importantly, theses studies are providing fundamental molecular insights into many biological, chemical, and physical phenomena, as well as changing the way we approach the synthesis of new molecular function.