Antibody engineering has received a boost from the development of an Escherichia coli expression system that now allows the screening of libraries with bacteria or phages. These random selection techniques can be applied using knowledge obtained from new X-ray structures of recombinant antibody domains, and anti-peptide antibodies. The first crystal structure of an anti-idiotype complex has also been solved. Additionally, the engineering of binding sites for metals and haptens, and the design of new immunotoxins have been reported.