Stability problems encountered in shale formations significantly increase the cost of drilling operations. It is not possible to identify a single main factor in shale instability; failure mechanisms are only now beginning to be identified, and the dominant processes seem to vary for different conditions of stress, mineralogy, fabric, and geochemistry. This article reviews a number of physical and chemical concepts in borehole stability analysis and raises several issues related to the activity coefficient and cation exchange processes. It is shown that the degree of saturation and clay particle separation have significant impact on the activity coefficient of tested cores. A simple coupling model based on the modified effective stress principle shows that when a shale with low porosity and high specific surface (reactive shale) is subjected to an increase in pore fluid salt concentration, a pore pressure front may develop. This front may trigger changes in effective stress and shear strength around the borehole. This may be a major destabilizing mechanism in the more reactive shales (generally the smectitic shales).