In the recent past, scientists wishing to use prokaryotic hosts to produce antibody domains have been presented with a staggering number of options. Many expression systems have been described which utilize unique plasmid vectors, regulated promoters, and host strains, and an amazing array of antibody domains including Fab, F(ab% single-chain antibody (SCA), and Fv. To the casual observer, this myriad of choices may seem daunting, but several important generalizations can be made. First, for most molecules of interest, a prokaryotic expression system can be identified that is useful for production. This statement may appear obvious, but until about 1985, mammalian lymphoid cells were the only source for antibody proteins. Second, antibody domains can be expressed both intracellularly in bacteria where they typically accumulate into insoluble inclusion bodies or they can be secreted from the bacterial cytoplasm where they often fold properly and can be recovered in an active form. Because each of these expression options presents some advantages (and disadvantages), planning is required to achieve optimal production goals.