This contribution deals with the derivation of mathematical models for continuous and batch crystallizers based on the population balance approach. Detailed kinetic expressions for primary nucleation, crystal growth and attrition are incorporated into the models. The proper mathematical formulation of these phenomena as well as their incorporation into the population balance are discussed. Subsequently, system theoretical properties (e.g. the differential index of the resulting differential-algebraic equation system) of the derived models are analysed. Finally, the models for the continuous and the batch crystallizer are validated by comparison to measurements of temperature, supersaturation and particle size distribution.