This paper presents a revision of some of the models that have been proposed to estimate the strength of unsaturated soils. The theoretical results obtained from these models are compared with the experimental results of controlled suction triaxial tests made on silty sand. The model that resulted in the best prediction was used to determine the stability of a slope of highly compressive clay subjected to changes in its water content. It is well known that the water content of soil is directly related with suction and that suction is an independent variable of the stress state that modifies the shear strength of unsaturated soils. Therefore, when the water content of a slope is modified, the safety factor of the slope is also modified. This exercise allows drawing some important conclusions over the stability of slopes related to the safety factor.