Beginning with the Escherichia coli— protein, or bacterial DNA topoisomerase I, an ever-increasing number of enzymes have been identified that catalyze changes in the linkage of DNA strands. DNA topoisomerases are ubiquitous in nature and have been shown to play critical roles in most processes involving DNA, including DNA replication, transcription, and rec- bination. These enzymes further constitute the cellular targets of a number of clinically important antibacterial and anticancer agents. Thus, further studies of DNA topology and DNA topoisomerases are critical to advance our und- standing of the basic biological processes required for cell cycle progression, cell division, genomic stability, and development. In addition, these studies will continue to provide critical insights into the cytotoxic action of drugs that target DNA topoisomerases. Such mechanistic studies have already played an important role in the development and clinical application of antimicrobial and chemotherapeutic agents. The two volumes of DNA Topoisomerase Prools are designed to help new and established researchers investigate all aspects of DNA topology and the function of these enzymes. The chapters are written by prominent investigators in the field and provide detailed background information and st- by-step experimental prools. The topics covered in Part I: DNA Topology and Enzymes, range from detailed methods to analyze various aspects of DNA structure, from linking number, knotting/unknotting, site-specific recombi- tion, and decatenation to the overexpression and purification of bacterial and eukaryotic DNA topoisomerases from a variety of cell systems and tissues.