Understanding the stress state in the earth's crust is crucial for engineers working in rock, particularly with regard to underground construction. Experience shows that an adequately high horizontal in-situ stress has a positive effect in stabilizing large span rock caverns close to the ground surface. On the other hand, high stresses resulting from large overburden, for example, may cause spalling and rock burst, threatening the integrity of the construction. The location, orientation and support design of underground structures takes into account the magnitude and orientation of in-situ rock stresses, considering such factors as gravity, topography, tectonics, residual stress, pore pressure change and geological structures. An accurate knowledge of in-situ rock stress can only be obtained by physical measurement, and over the last few years there has been substantial development in techniques and in interpretation of the results. The papers in this volume will be of particular interest to those working in tunnelling and mining and in petroleum exploration and production.