Three themes are developed in this paper. The first deals with those principles basic to the science of geology and thus to the use of geology in geotechnical engineering; namely, the principles of superposition and uniformitarianism; their use is illustrated with reference to commonly occurring geotechnical situations. Then the application of these principles in practice is explained, together with the difficulties that follow if little thought is given to the demands they make for their proper use. Two common causes for difficulty are considered; namely, using an inappropriate description and working at an inappropriate scale. Finally, the potential geology has to open new areas of research is illustrated with examples, ranging in dimension over almost 14 orders of magnitude, the largest involving regional structure, the London Basin. The examples that follow are of progressively smaller volume: displacements in landslides, the failure of crystalline rock, the response of mineral surfaces to contact with water, the possible effects of strain energy stored in minerals, and the characters of silica gel, a material that could influence and possibly govern the geomechanical and geochemical response of silicate particles. Each of these subjects could lead to an understanding of geological materials that changes the development of geotechnical engineering. Each is capable of becoming a scientific area to which engineers refer for solutions to problems in ground engineering and guidance in the use of soils as a construction material. In developing these themes mention is also made of the roots of engineering geology in Britain, as these throw light onto the similarities and differences between engineering geology and geology for engineers, and between engineering geology used in practice and that needed to develop the subject. The greatest danger at present comes from the lack of education in practical and theoretical geology for many who enter engineering geology; it is that which will do most to limit the extraordinary potential for advances in geotechnical engineering that can be provided by geology.