The structures of several Fab fragments and Fab-antigen complexes have now been solved at high resolution. These structures of antibodies in complex with proteins, peptides and various other haptens have enabled us to gain insights into the structural basis of immune recognition. Early structures of Fab fragments with and without bound haptens showed the antibody combining sites to be pockets or grooves. More recent Fab-protein complex structures have shown the antibody-antigen interactions to be more extensive with flatter, more undulating binding surfaces. We have solved the structures of three Fab fragments in their native form and as complexes with their respective antigens. Two of these are anti-peptide Fab fragments, the other an anti-progesterone Fab. Comparison of the free and bound structures indicates small but significant changes in the antibody on ligand binding. An analysis of the Fab complexes solved so far indicates that the antibodies can have very differently shaped binding sites, depending on the antigen.