The first libraries of complementary DNA (cDNA) clones were con- structed in the mid-to-late 1970s using RNA-dependent DNA polymerase (reverse transcriptase) to convert poly A* mRNA into double-stranded cDNA suitable for insertion into prokaryotic vectors. Since then cDNA technology has become a fundamental tool for the molecular biologist and at the same time some very significant advances have occurred in the methods for con- structing and screening cDNA libraries. It is not the aim of cDNA Library Protocols to give a comprehensive review of all cDNA library-based methodologies; instead we present a series of up-to-date protocols that together should give a good grounding of proce- dures associated with the construction and use of cDNA libraries. In deciding what to include, we endeavored to combine up-to-date versions of some of the most widely used protocols with some very usefiil newer techniques. cDNA Library Protocols should therefore be especially useful to the investigator who is new to the use of cDNA libraries, but should also be of value to the more experienced worker. Chapters 1--5 concentrate on cDNA library construction and manipula- tion, Chapters 6 and 7 describe means of cloning difficult-to-obtain ends of cDNAs, Chapters 8-18 give various approaches to the screening of cDNA libraries, and the remaining chapters present methods of analysis of cDNA clones including details of how to analyze cDNA sequence data and how to make use of the wealth of cDNA data emerging from the human genome project.