Summary The term facies is used either descriptively, for a certain volume of sediment, or interpretatively for the inferred depositional environment of that sediment. Facies models are intellectual aids to the understanding of sedimentary environments and the origin of ancient sedimentary rocks. Many different models can be constructed to explain a given set of data, depending on which aspect of the facies requires the most illumination and on the types of techniques used in the analysis. When dealing with ancient rocks, facies modelling is the last stage in the process of facies analysis which consists of the detailed description of exposed or cored sediments, their classification into objectively defined facies, the compilation of the characteristics of each facies, the deduction of the processes of deposition of each facies, the examination of the spatial relationships between facies and the recognition of facies associations, the interpretation of the overall depositional environment of the association and the detailed interpretation and modelling of individual facies. Short cuts can lead to fallacious interpretations. Reliable facies models cannot be produced without careful facies analysis. A facies model may be plausible, yet totally inapplicable to the facies from which it was supposedly derived.