This survey of literature was undertaken by ARTEP - the French Research Association for Oil Exploration and Production Techniques - at the begin ni ng of STAR (=STabilite des ARgiles), a project on the influence of clays on borehole stability. Knowledge of theories and laboratory experiments was indeed felt very necessary to help understanding, and thus becoming able to prevent, quite damaging phenomena. During the time spent on this project, ideas and interpretations of all participants underwent some evolution due to the comparison between theories and experiment, and new procedures and interpretations are being proposed elsewhere. The survey is divided in tour sections : The first section recalls the specific problems caused by the occurrence of shales during drilling operations for oil or gas : about 90 % of the problems, for about 70% of the drilled formations. The behaviour of the shales leads to a classification in four different classes : dispersive, swelling, heaving and brittle. They are spread all over the world, not only an the Gulf Coast of the USA and in the North Sea, where they have been more extensively studied, but also in former USSR, in Asia and Africa. Due to deposit conditions and diagenetic history, they occur at different depths, with different properties : reactive shales, at shallow depths, under-, over- or normally-compacted formations, and reservoir caps. Besides their mineralogical and textural properties, due to the large proportion of clays, they have damaging properties as the low permeability (10(-6) to 10(-12) D). The variety of reactions with water extends from a complete dispersion in mud to cavings or swelling of the borehole, with cuttings ranging from less than 1 mm to more than several cm. This has led to use of various empirical solutions to protect the borehole, with mainly mechanical or chemical objectives. However, it is felt that the general solutions can arise only from a synergistic effort of both rock mechanics and physico-chemistry, hence the STAR program. The second section is devoted to a survey of physico-chemical reactions between clays and water. It begins with some definitions of the clays as solids, from the rock to the atomic level. All clays are characterised by their small size, in the range of 1 mu m, and thus, their high surface area: from one to several hundred m(2)/g. Existence of a layer charge, and of compensating cations is the key of the behaviour of the swelling clays (i.e. smectites) versus water, pure or with cations, which deserve particular attention. Description of the various kinds of water associated, more or less energetically, with days, as a function of relative humidity RH, or water activity aw, helps to distinguish their effect on porosity and texture. Hydration and dehydration behaviour, with a particular hysteresis, is described, in solid-gas systems as well as in solid-liquid, closer to field conditions. Two main domains are distinguished: crystalline swelling, up to a water content of circa 50 vol.-%, inducing swelling pressures in the range of several thousand atmospheres, and osmotic swelling, for higher water contents, inducing pressures in the range of several atmospheres. influence of nature and amount of cations is examined, mainly for calcium, which induces limited swelling, and for potassium which is both less hydratable and proner to irreversible fixation on clay. Little work has already been performed on the influence of temperature and pressure. Mechanisms of water and/or ionic species transport are reviewed : diffusion and osmosis, without applied pressure, and effect of pressure. Behaviour on compaction, always showing hysteresis, depends on the nature of the clays, and of the cations, but also on the composition of the solution. It is described using suction pressures or mechanical stresses, which induce different properties of the final solid. Experiments in soil literature, generally performed in the presence of a gas phase, cannot be readily compared to the in situ behaviour of the shales, but give insights on the possible artefacts of laboratory experiments. Caution is thus necessary before any application of literature results to real samples, all preliminary conditioning (initial state and composition of the clay and the water, way of hydration/dehydration, or compaction) being able to modify the behaviour of the clay-water system. The third section sets the problem of describing the mechanical behaviour of the rock formation on drilling. This behaviour depends on initial in situ stresses, pore pressure and temperature, and on the constitutive law of the rock, i.e, the relation between stress and strain. As an example, the Cam Clay elasto-plastic law is developed. Then the laboratory experimental sets used to identify mechanical properties are described: triaxial tests, drained or undrained, oedometric tests, and hollow cylinder tests, the first ones being used to calibrate borehole stability, while the latter simulate drilled boreholes. Specific aspects of shares are then recalled: dependence of mechanical properties on the water content, anisotropy and influence of time. Coupling between physico-chemistry and mechanics arises from the lack of chemical equilibrium between the solid and the liquid. This desequilibrium induces a transfer of water and chemical species in solution, modifying the pore pressure, thus the stress on the rock, and leading to chemical reactions, which have been described in section II. Follows a description of stability models, which should be able to predict mud characteristics for the drilling as well as evolution of the borehole with time. Stability models intend to calculate the maximum/minimum mud weight, from a relevant instability criterion, drawn from well data, mechanical data and fluid properties. The choice of the constitutive law is thus important, and elasto-plastic ones seem the more relevant. Taking into account physico-chemistry has been done generally using an ＇＇osmotic＇＇ pressure, with the assumption that the shares behave as a semi-permeable membrane. Even if this assumption is too simplistic, it is still used, but more refined models are being studied, which take into account variations in pore pressures and salinities. The fourth section deals with what actually occurs on application to real wellbore. Improvement of mud formulation tries to prevent any problem occurring during drilling. Evolution of formulation is described, from lime, oil, KCl to polymers additions, and to nowadays constraints brought by environment concerns : requirements may be opposite, and thus compromises must be found. Mud monitoring seems a good prospect. In a second part, availability of representative samples, artefacts related to the recovery and storage of samples as well as choice of experimental conditions are reviewed. It is recalled that downhole conditions are rarely taken into account, and problems like drying of the samples, which induces a suction pressure, anisotropy and cohesion of the samples are rarely considered. The conclusion emphasises that if swelling pressures can now be more precisely defined in hydration domains, the effects of physicochemistry on mechanical properties are still to be investigated more thoroughly. Further experiments should be set in conditions closer to downhole ones, and teams must work together to get ail the data needed. This will allow proper stability modelling, with coupling of physico-chemistry and mechanics, to become a predictive tool.