Continuous changes occur in the coiling, supercoiling, and catenation of DNA molecules during gene transcription and cell division. The activity of DNA is thus heavily influenced by its topology, yet this influence is not widely appreciated or understood. This book explains the biological implications of DNA superstructure for readers who are presumed not to know higher mathematics. It is an introductory text, but not an elementary one, and will be valuable for molecular biologists at graduate student level and beyond.