DNA topoisomerases are ubiquitous enzymes that control the level of supercoiling of DNA in cells. There are several classes, each with distinct properties, which are briefly discussed in this review. High-resolution X-ray crystallographic structures have been obtained for fragments of two classes of these enzymes, which when combined with biochemical data, reveal a great deal about the gymnastics that the enzymes undergo during catalysis and provide fascinating snapshots of their mechanisms. These mechanisms are discussed in detail. Finally, the first structure of a topoisomerase in a complex with an antibiotic was recently solved. This structure is briefly discussed with regard to the biochemical activity of the compound.