In this review we evaluate existing experimental methods for the electrophoretic size separation of nucleic acids, including both slab gel electrophoresis and capillary electrophoresis (CE). First, we briefly discuss some important applications of DNA electrophoresis in molecular biology. We then focus on the theory and the practice of DNA separations by slab gel electrophoresis and CE, emphasizing the different practical limitations of the two techniques. In particular, we review new developments in CE, a technique which is evolving rapidly.