Various types of final clarifier model are introduced. The approaches and simplifications used for one-dimensional modelling are outlined, whilst new results of hydrodynamic modelling demonstrate how complex the flow field and the interactions with the settling behaviour of the sludge are. Based on the examination of which processes and boundary conditions affect the clarifier performance, what basic knowledge is lacking for modelling is analysed. One- and two-dimensional models are assessed with respect to their capacities and their shortcomings. Finally, the promising applications of the various models are pointed out, and future research potential for significant improvement of modelling is indicated.