Summary: Previous experiments on natural convection in a differentially heated porous layer with large lateral dimensions gave evidence for different configurations of flow. Depending on the values of the Rayleigh number, the inclination and the longitudinal extension of the layer, the three main structures observed correspond to a two-dimensional unicellular flow, polyhedral convective cells and longitudinal coils. In this paper there is a definition of the conditions necessary for these types of flow to exist using a linear stability theory and it is shown that the experimentally observed structures can be theoretically predicted by a three-dimensional numerical model based upon Galerkin’s spectral method. Finally, the results of this model are used to show the influence of initial conditions on the setting up of the stationary flow.