Bacteriophage λ has been in use as a cloning vector for over 20 years, and has been extensively used as an expression vector. A historical overview of λ as a cloning vector can be found in Murray (1 ), and a more recent review of λ vectors is in Chauthaiwale et al. (2 ). In general, λ is more useful as a tool for expressing foreign genes when a library is to be screened. For high level production and purification of a particular foreign protein in E. coli , plasmids are generally preferred, although λ vectors can be used. There are also λ vectors that can readily be converted into plasmids for production of foreign proteins. The purpose of this chapter is to review bacteriophage λ based expression vectors and their features as an aid in choosing a vector or in understanding the features of a particular λ library or clone. Detailed methods on cDNA library construction including mRNA isolation, cDNA synthesis, ligation, plating and screening λ libraries with antibodies or DNA probes can be found in a number of cloning manuals (3 ). In addition, several λ vectors are commercially available either by themselves or as part of cDNA cloning kits with detailed instructions.