The technique of drilling boreholes using air-flush rotary-percussive drilling rigs is widely used in the quarrying and civil engineering industries, but its application in site investigation does not appear to have been appreciated. It involves the drilling of boreholes under controlled conditions, using a rotary-percussive drill, and measuring the rate of penetration. Systematic sampling of the drilling flushings and observation of the drilling characteristics enable an indentification of the rock type to be made, as well as an interpretation of some important aspects of its geotechnical behaviour. Although the technique does not provide a continuous visual record of the intact material drilled, it has been extensively and economically applied in South-west England, particularly in the search for cavities in rocks and the depths to, and quality of, bedrock and variations in the properties of some types of soils. The interpretation of the results is further improved if the results of some core-drilled or shell-and-auger drilled boreholes are available for direct comparison, correlation and “calibration”. The technique can also be applied in cases where the boreholes are drilled for other purposes, such as grouting, rock anchoring and blast-hole drilling, in order to supplement knowledge of the ground conditions.