It has been known for a long time now that in soft and sensitive clays undrained failure occurs apparently before the full friction angle has been mobilized. The main object of this paper is to clarify the importance of deformations in the process of mobilizing undrained shear strength. This has resulted in an improved understanding of the failure process, and made it possible to express undrained shear strength for this type of clays in terms of effective stresses and a set of effective shear strength parameters, friction and attraction. The study made it possible to determine the relationship between the coefficient of earth pressure at rest K0, the undrained shear strength su and the overconsolidation ratio OCR during cycles of consolidation, unloading and reloading. The determinations, done both analytically and graphically, were done for a typical low plasticity clay, and the results compared with published data. For comparison, laboratory test results for a somewhat stiffer and dilating offshore clay are also included in this paper.