Recombinant antibody technology is a rapidly evolving field that enables the study and improvement of antibody properties by means of genetic engineering. Moreover, the functional expression of antibody fragments in Escherichia coli has formed the basis for antibody library generation and selection, a powerful method to produce human antibodies for therapy. Because in vitro-generated antibodies offer various advantages over traditionally produced monoclonal antibodies, such molecules are now increasingly used for standard immunological assays. This chapter will give a short review on how recombinant antibodies are generally be produced and engineered, and how typical immunoassays are performed.