The role and historical progress of n.m.r. applications in biochemistry are briefly outlined. Technical advances over the years have made n.m.r., at last, a technique which can give valuable information about a wide range of biochemical topics, from enzyme kinetics in vivo to the structure of protein-DNA complexes. Emphasis here is placed on studies of proteins, especially those made up from mosaics of modules. It is shown that n.m.r. can readily give detailed structural information about individual protein modules and that valuable information about the structure and function of the intact mosaic protein can be inferred.