Separation of DNA by size is at the heart of genome mapping and sequencing and is likely to play an increasing role in diagnosis. This study reviews the mechanism responsible for electrophoretic separation of polyelectrolytes. This separation is mainly performed in gels, and wide variety of migration mechanisms can come into play, depending on the polyelectrolytes's architecture, on the electric fields applied, and on the properties of the gell. The reptation mechanisms by which large flexible polyelectrolytes thread their way through the pores of the gel matrix, play a prominent role.