The theory of capillary electrophoresis with polymeric separation media is reviewed. Some of the major theories of gel electrophoresis are examined in terms of the experimental observations that they support. The considerations and difficulties in applying these theories to capillary electrophoresis with polymeric separation media are investigated, particularly the use of high electric field strengths and low concentration polymer solutions. Recent advances in this area, including some empirically-derived relationships and models currently found in the literature, and future directions for theory development are discussed.