A phenomenological theory of charge and/or spin density waves at surfaces is presented. The density wave is described by an order parameter which obeys an equation similar to that proposed by Ginzburg and Landau for superconductivity. A condition is obtained for the existence of density waves in terms of two parameters. These are computed for a simple model antiferromagnet and the results are used to estimate the transition temperature of surface antiferromagnetism in chromium and chromium alloys. Other materials are also discussed.