An improved understanding of the molecular processes of migration and reorientation of DNA molecules undergoing conventional and pulsed-field gel electrophoresis (PFGE) should provide the basis for the rational optimization of these techniques. This review of common pulsed-field methods emphasizes the range of their applications as well as their current limitations. The molecular mechanisms involved in electrophoresis, as they are beginning to emerge from a combination of direct observations using a microscope and computer-modeling studies, are also discussed. © 1993.