This paper reports systematic studies on hydrodynamic stress in agitated vessels with baffles, reactors with predominantly laminar flow, SERALE viscosimeters, bubble columns, and gas-driven loop reactors. The stress on particles is determined by way of the destruction of model particle systems, with the kinetics of destruction, the equilibrium particle diameters, and the enzyme activity after a given time serving, respectively, for assessment of the stress. Thereof one obtaines an indication of the methodology necessary for determining the stress and it permits selection of appropriate reactor systems according to the criterion of particle stress. In the case of reactors with turbulent motion without dominant laminar flow or gas/liquid interfaces give analogous results, permitting the assumption that they are also applicable to other particle systems. Specifically for stirred reactors a geometric function is derived from the experimental results which permits prediction of stress caused by various types of impellers. Impellers with large blade areas relative to the dimensions of the tank produce less shearing forces owing to their uniform power input, in contrast to small and especially axial flow impellers. In laminar flow or reactors with gas/liquid interfaces the level of particle stress depends upon the proclivity of the particles to enter the boundary layers. It can therefore be deduced that particles with different surface properties, which lead to differing interactions with boundary layers and interfaces, will also experience different kinds of particle stress on laminar flow or in aeration processes. Thus the scope for application of the results obtained with model particle systems to other particle systems is limited in such reactors.