Most of the unit operations of chemical engineering are based on disperse systems, i.e. particles, droplets or gas bubbles. To improve the description of these unit operations, it is necessary to look at the microscopic processes and to introduce population balances as a tool for handling the complex interactions within disperse systems. It is shown that the progress in computer and software technology allows an effective modelling and simulation of unit operations. We expect in the near future models which are applicable in daily work. This will lead to a more effective design of processes and process units based on lab-scale experiments.