The scale-up of fluidized-bed processes involves not only the enlargement of the size of the reactor but also characteristic changes of design details. A two-phase model together with a comprehensive fluid-mechanical description of bubbling fluidized beds is presented which is shown to be helpful in the estimation of scale-up effects on the operating behavior of fluidized bed reactors. The effects of bed diameter, distributor design, baffles and mean bed particle size, respectively, are considered. Model calculations are compared to measurements which either have been carried out by the author's group or were taken from the literature.