It has been almost 100 years since von Behring and Kitasato received the first Nobel prize for the discovery of passive immunotherapy and nearly 25 years since Köhler and Milstein first reported hybridoma technology. In the 15 years since Mullis and co-workers described PCR, a number of discoveries and technologies have converged to produce a renaissance in antibody therapeutics. Our vision of antibodies as tools for research--useful for the prevention, detection and treatment of disease--has been revolutionized by these recent advances. This review specifically focuses on what is now called antibody engineering and includes chimeric and humanized antibodies, immunoglobulin fragments, antibody libraries, antibody fusion proteins and transgenic organisms as bioreactors. As a consequence of refinements in antibody technology, the field of genetically engineered immunoglobulins has matured into an elegant and important drug and reagent development platform.