Marine sand waves and river dunes are rhythmic patterns, formed at the interface between a sandy bed and turbulent flows, driven by tidal and river currents. Their origin, development, and dynamics are far from being fully understood. To enhance the discussion on this topic we organized the international conference Marine Sand Wave and River Dune Dynamics (MARID) in April 2004 in Enschede, Netherlands. Scientific papers that originated from the MARID conference are presented in this special section. This introduction places these papers in the context of recent progress.