In the framework of project Strength evolution of soft consolidating mud layers, financially supported by the Netherlands Foundation of Technology, a literature survey on consolidation theory and rheological modelling of mud was carried out. A consolidation theory, focused on the Gibson equation (Gibson et al., 1967), is presented. The solution of Gibson's one-dimensional finite strain consolidation equation requires closure relations for the effective stress and the permeability. It is assumed that both relations are functions of the void ratio only. However, the consolidation of soft soils cannot be modelled adequately with these assumptions, since effects as aging, channeling, creep, formation and break-up of flocs, for example, are not taken into account. The incorporation of a rheological model into the consolidation model can result in a better modelling of consolidating mud layers. From this point of view, various rheological models are presented, starting with simple models that become increasingly complex. The complex models incorporate thixotropy and yield stress to describe rheolocial complex fluids as mud. Experimental evidence of a relationship between effective stress and yield stress is presented. Topics for future research are indicated.