DNA gyrase is an essential bacterial enzyme that catalyzes the ATP-dependent negative super-coiling of double-stranded closed-circular DNA. Gyrase belongs to a class of enzymes known as topoisomerases that are involved in the control of topological transitions of DNA. The mechanism by which gyrase is able to influence the topological state of DNA molecules is of inherent interest from an enzymological standpoint. In addition, much attention has been focused on DNA gyrase as the intracellular target of a number of antibacterial agents as a paradigm for other DNA topoisomerases. In this review we summarize the current knowledge concerning DNA gyrase by addressing a wide range of aspects of the study of this enzyme.