In this paper the importance of on-scale crystalliser design is outlined. An on-scale approach is specifically required for the analysis and optimisation tasks in design. The need for this approach is a direct consequence of the nonlinear dependency of most physical processes in crystallisation on the degree of saturation, the energy dissipation, the crystal size and its distribution. The hydrodynamics in a crystalliser vessel are typically such, that these process variables are distributed non-uniformly throughout the vessel. The conventional, geometrically lumped description of the physical processes inside a crystalliser vessel, i.e. nucleation; growth, dissolution, attrition,;breakage, agglomeration and particle segregation, has therefore never proven-to be reliable for scale-up purposes. Furthermore, as the interactions between these processes lead to an intricate dynamic behaviour, models describing the effect of changes in time of process variables on the product quality are essential. Compartmental modelling, a well known technique in reactor engineering and applied within crystallisation since a number of years, facilitates on-scale/design since it allows a natural separation of kinetic and hydrodynamic mechanisms. The-resulting dynamic models (order of 10(4) equations) can be easily tackled with standard DAE solvers.Here we will focus upon the need for a proper physical description of the aforementioned crystallisation mechanisms. First of all, a brief description of the dependencies of these mechanisms upon local supersaturation or undersaturation, local energy dissipation and crystal size is given. Depending on the type of crystallisation process, suspension crystallisation or precipitation, the dependencies necessary to be included in the compartmental model, in order to describe their overall effect are discussed. The next step is deriving the geometric structure of a compartmental model for a certain scale crystalliser and material, for which two methodologies will be presented. Finally, the approach will be illustrated for evaporative crystallisation of ammonium sulphate from water in 0.15 and 18.5 m(3) FC (Forced Circulation) and 0.022 and 1.1 m(3) DTB (Draft Tube Baffle) crystallisers, using size dependent nucleation, growth, dissolution, attrition and segregation models.