The current approaches to modelling the groundwater resources of small islands are briefly reviewed and the importance of representing the variation in salinity through a transition zone between the freshwater lens and sea water is emphasized. Older approaches based on a sharp interface approximation are inappropriate. The current modelling approach is based on the advection–dispersion equation. However, key parameters in this equation are scale, density and problem specific, making the approach difficult to use predictively. The density dependence is illustrated by simple analytical results and experiments, and future consideration of this dependence is advocated. Alternative modelling approaches are identified, and investigation of their usefulness is recommended.