This document presents information on genomic and cDNA libraries. It begins by defining a gene library as a collection of DNA sequences from an organism cloned into a vector. There are two main types - genomic libraries containing all sequences from the genome, and cDNA libraries containing sequences represented in mRNA. The document then describes the principles, vectors, and procedures used to construct each type of library. Key steps include fragmenting genomic DNA, ligating fragments into vectors, and amplifying the libraries in host cells for genomic libraries, and reverse transcribing mRNA and ligating cDNA into vectors for cDNA libraries. The advantages and disadvantages of each approach are also summarized.