Fresh groundwater in low carbonate islands normally occurs within the upper unconsolidated sediments as a lens-shaped body that is buoyantly supported by dense underlying saline water. The thickness of a freshwater lens generally depends on the recharge rate, the width of the island, the hydraulic conductivity of the sediments, the depth of the Thurber Discontinuity, and the presence (or absence) of a reef flat plate. Various examples from the Pacific are presented where recent investigations using Electrical Resistivity Tomography for the quantification of fresh groundwater resources have indeed revealed that island width alone is not enough of an indication for the thickness of a freshwater lens. In Tuvalu, for example, resistivity responses along relatively wide islands (Vaitupu, Motulalo) have indicated the presence of limited groundwater whereas resistivity models along smaller islands (Fale, Lakena) have suggested the presence of significantly thick freshwater lenses with high development potential for potable and other domestic purposes. High resolution profiles depicting the spatial distribution in the resistivity of subsurface media can reveal patterns and provide insights on the genesis and evolution of carbonate island hydrogeology/geomorphology and freshwater lens development. The value of drilling logs and groundwater monitoring bores is once again highlighted as a means of calibrating resistivity results and extrapolating along modelled survey profiles.