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In the years 1991 and 1992 combined vertical electric (VES) and electromagnetic (EM) soundings, complemented by chemical analyses of borehole water samples, were made with the ahn of detecting and mapping saline-fresh water interfaces in coastal and estuarine sediments. Two coastal areas, one at Morfa Bychan at the northern edge of Cardigan Bay and another on the College Farm at Aber on the Menai Strait, and one estuarine area on the northern bank of the Mon Cefiii at Malltraeth, were chosen for detailed study. The VES and EM data show close agreement to themselves as well as to the salinity of the groundwater determined by chemical means. On the basis of these and borehole data it has been possible to define an aquifer at each site, boundaries of saline intrusion into groundwater, and zones of mixing between the two. An aquifer filled with fresh groundwater has a resistivity of more than 35 ohm. m (chlorides less than 250 ppm), the zone of mixing water (transition zone) has resistivities ranging between 8 ohm. rn to 35 ohm. m (chlorides between 250 to 500 ppm), and saline water has a resistivity of 7 ohm. m or less (chlorides 500 pprn or more). These data cast some doubt on the applicability of the Ghijben-Herzberg Relationship in defining the position of the saline-fresh water interface. In addition to these observations, pumping tests were carried out which not only gave permeability values but also showed the modification of the saline interface caused by pumping. Permeabilities were also measured both in-situ and in the laboratory on collected samples using standard techniques; permeabilities were also estimated using electrical formation factor considerations. In general close similarities existed between the various techniques. The permeabilities of the aquifers range from 3.2 x 10' rn /sec to 1.3 x 10' m/sec which can be classified as moderate. It is clear that in the sites examined a combination of permeability and an abundance of fresh water from the land (from rainfall and rivers) prevent any extensive saline intrusion. However the intrusive zone is extended during the period when spring tides operate. The overall conclusion is that VES and EM techniques are dependable practical tools for the determination and mapping of any saline intrusion. Importantly they can also provide values of porosity and permeability which agree closely with those parameters obtained by traditional hydrological methods. This opens a way forward for the use of electrical techniques as precursors to any detailed investigation such as pumping tests. They can also be used in a monitoring mode for detecting changes in aquifer salinity caused by abstraction of groundwater allowing saline intrusion to occur. Such a procedure can be of considerable significance in Pakistan to assess the severity of the twin problems of waterlogging and commensurate salinity changes.