The development of the Bacterial Artificial Chromosome (BAC) system was driven in part by the Human Genome Project as a means to construct genomic DNA libraries and physical maps for genomic sequencing. The BAC system is based on the well-characterized Escherichia coli F-factor, a low copy plasmid that exists in a supercoiled circular form in host cells. The structural features of the F-factor allow stable maintenance of individual human DNA clones as well as easymanipulation of the cloned DNA. BACs are currently used in a wide array of applications from genome sequencing to gene discovery. This paper describes the key elements in the development of the BAC system and its current notable applications.