In this presentation, we discuss geophysical methods for general vulnerability mapping of aquifers. The vulnerability of aquifers is to a large degree determined by the content of clay minerals in the near surface geology. Clay minerals have a large cation exchange capacity (CEC) and clayey sediments reduce the infiltration speed thus reducing the aquifer vulnerability. In sedimentary areas, the near surface geology often shows large variations, and therefore the requirements for dense detailed geophysical data are required in order to achieve a satisfying geological resolution. The DC resistivity method is a good choice as it has a high near surface resolution. Alternatively, very early transient electromagnetic measurements can be used. The latter are cost effective and can often be made from the air. We discuss advantages and drawbacks of the two methods and show an example from a 100 km survey where we have used a newly developed variation of the helicopter-borne SkyTEM system for vulnerability mapping. Both the Zand the X-component of the decaying field were measured and the data were inverted using a laterally constrained inversion method.