The diffusive regime of double-diffusive convection is discussed, with a particular focus on unresolved issues that are holding up the development of large-scale parameterizations. Some of these issues, such as interfacial transports and layer-interface interactions, may be studied in isolation. Laboratory work should help with these. However, we must also face more difficult matters that relate to oceanic phenomena that are not represented easily in the laboratory. These lie beneath some fundamental questions about how double-diffusive structures are formed in the ocean, and how they evolve in the competitive ocean environment. (C) 2003 Elsevier Science Ltd. All rights reserved.