In product and process design, particle system characterization and modelling are both important tools to understand processes and product behaviour. Recent experience shows that there is a very close reciprocal dependence of the two activities on each other. In a first step it was realized that in solids processes such as grinding many product and equipment properties have to be measured by experiments. Off-line and on-line characterization methods were employed and preparation techniques for the samples were developed. In a second step, grinding models from the literature were tested, and it was found that one could achieve reasonable but not fully satisfactory agreement. The conclusions drawn were that both models and characterization techniques have to be improved. These findings have already been discussed in previous papers. At present, the on-line methods are well suited for process control and to detect transient behaviour. Their precision is not sufficient for the required depth of information, so that laboratory methods also have to be employed. In this paper, it is pointed out that this process is something like an iteration consisting of several steps. The modelling capabilities achieved are a very effective instrument to test the validity of the characterization methods and to assess the effect of the preparation methods. One has to consider three categories of models: - a model which describes the dependence of product quality on the disperse state and behaviour, - a model which describes the process conditions leading to the disperse state, and - a model which describes the physical measuring effect in a complex particle system. These models are very closely related. Measurements were carried out to elucidate the product behaviour and the process, but one must understand the product behaviour and the process in order to carry out the measurements correctly. This concept is discussed with some examples taken from dry and wet grinding.