Both deterministic and probabilistic approaches are often used to carry out analyses of individual slopes and to estimate values of the performance indicators such as 'factor of safety', 'reliability index' or 'probability of failure'. However, the back-analysis of the observed performance of a slope is always carried out on a deterministic basis. In such analyses, however, the variability of geotechnical parameters cannot be included directly nor can systematic uncertainties be accounted for. Therefore, probabilistic back-analyses would be desirable. However, an attempt to develop a proper procedure for such analyses encounters both conceptual and practical difficulties. In this paper, therefore, it is proposed to embrace the reliability updating approach and a procedure is outlined for achieving this. This can be applied to a failure case as well as to a survival case. It involves consideration of conditional probabilities. A well documented case of a failed slope is reconsidered on the basis of the proposed approach. It is shown that the calculated values of performance indicators and geotechnical parameters are credible and realistic and that the results are a vast improvement on values obtained without the updating procedure. For the covering abstract see ITRD E128041.