The growing trends in tailings management are to eliminate ponded water on top of tailings and to thicken tailings to conserve water and minimise the amount of water that has to be managed. There are various degrees of thickening ranging from a high density, non-segregated slurry, to a paste and filtered tailings.Thickened tailings, with a consistency of a high density, non-segregated slurry or a paste can be safely deposited at slopes between 2-10% in thin layers that allow for desiccation with relatively small containment dams at the toe. However, stability is a concern, especially for large tailings deposits because the deposited properties can not accurately be predicted beforehand. Empirical relationships have not been developed because there is insufficient field data. This paper discusses the stability of thickened tailings stacks (slopes) in both the short-term during operation and long-term in the closure mode. The authors have developed an innovative design methodology based on critical state soil mechanics fundamentals that can predict the stability of thickened tailings slopes and the change in the factor of safety with time. This tool can also be used to carry out sensitivity analyses for different operational conditions and different tailings properties so that deposition strategies can be optimised. Thin layer deposition and desiccation are necessary to ensure the stability of these slopes. A feature of the method is that the deviations from the design assumptions are evident shortly after deposition commences allowing time to implement changes.