Ground-penetrating-radar (GPR) has been used for nondestructive testing of concrete and other building materials. A number of papers have dealt with the use of GPR in detecting voids, fractures, and internal components of masonry. The two case studies discussed here show that GPR using a 1,500 MHZ antenna is capable of detecting deteriorated mortar, as well as voids. Some of the detected voids may have been created during construction, although dissolution of any calcium based mortar will eventually lead to elution of soluble components and dis aggregation of the mortar. GPR is a reflection technique, much like a sonar-based depth finder. It is not a transmission system, such as X-ray, which requires an emitter of gamma radiation and a photographic plate or screen on the other side of a subject that will receive the transmitted images.