The collapse of an old masonry wall on Hong Kong Island in 1994 prompted research by the Hong Kong SAR Government into the use of modern, non-invasive, geophysical investigative techniques for site characterization. Hong Kong has thousands of retaining walls and during periods of high rainfall, some old masonry walls have failed, damaging property, restricting access and occasionally leading to loss of life. Occasional catastrophic failure of old masonry walls has been linked to combinations of lack of design, high pore pressures, leaking utilities, substandard construction, void development and changes in the land use around the structure. In Hong Kong, the identification of anomalous features in or behind a wall, using conventional investigations such as drilling and trial pits, is expensive and time consuming. Because of the discrete nature of these intrusive methods, they are also less likely than geophysical methods to intersect anomalous features that could adversely affect the stability of a masonry wall. One of the main objectives of the research was to identify efficient, non-invasive, geophysical tools that could assess the geometry and structure of old masonry walls, provide information on the hydrogeological conditions and continuous images of the subsurface and, if required, guide conventional investigation methods. The main conclusion is that a combination of ground penetrating radar and electrical imaging has the potential to identify ‘thin’ old masonry retaining walls in Hong Kong provided that attention to detail during data acquisition and the correct application of analytical techniques is made.