Sub-surface impulse radar has recently become an established technique for detecting a wide range of buried features in concrete. The background and scope of this technique are outlined in this paper, together with relevant findings of a recent major 5. E. R. C. funded project undertaken at Liverpool University. Location and identification of buried steel reinforcing bars and voids represent two important applications which are considered, with particular attention concentrated upon voids. A wide range of experimental results have been obtained for various configurations of size, shape and depth and are supported by numerical modelling. The ability of a I GHz system to detect air and water filled voids is discussed in detail, and practical guidance is offered to assist the execution and interpretation of such investigations.