This chapter describes the processes which produce density structure in shelf seas, and the scales which these determine. This shows that high resolution models are necessary, with spatial scales of kilometers, and covering time development of features over several days. The chapter compares density-driven flow with the other processes causing flows in shelf seas. It focuses on the density-driven flow alone. Of course, some secondary effects of other flows, such as tidal mixing, are likely to be important. The chapter discusses the possible numerical techniques for modeling density-driven currents, reviews some models, and includes some speculation about future trends in an era of rapidly increasing computer power and developing measurement techniques. North Sea applications of the Backhaus model have included the density-driven circulation implied by seasonal data in a diagnostic manner, except that the model was run in a prognostic mode to interpolate the density data dynamically in order to achieve a realistic, balanced distribution.