The accumulated experience on slope stability is reviewed with due consideration to the time-dependent behaviour of natural clays prior to failure. A consistent picture of the behaviour of both man-made and natural slopes is obtained, which applies to soft as well as stiff clays, in an intact or a fissured state. In all cases, creep deformations develop prior to failure, which can be used as an indicator of incipient failure. The strength mobilized at failure can be theoretically defined in a qualitative manner: it is intermediate to the critical state strength and the time-dependent limit state of the clay. Quantitatively, this strength is represented by thevalue corresponding to the critical state and by an empirical cohesive term in the order of 4–15 kPa in intact clays and of 0–2 kPa in fissured clays; these values apply to both cuttings and natural slopes.