The scale-up of stirred tanks to very large size is challenging because measurements and correlations are mainly developed for small-scale apparatus. This paper presents the basic hydrodynamics of two-phase stirred tanks. The hydrodynamics in large-scale reactors are shown to be mainly a function of the superficial gas velocity in the system. For this reason scale-up by keeping the superficial gas velocity constant is suggested in comparison to scale-up by constant volumetric aeration per volume (vvm). It is shown that in order to achieve adequate mass transfer, large-scale stirred tanks, especially bioreactors, must operate mainly in the heterogeneous regime and therefore the correlations developed in small-scale and homogeneous regime are not directly applicable for scale-up. New simple ways to predict gas holdup, interfacial area and mass transfer in stirred tanks are presented and shown to follow experimental values. The model requires very little data as basis and can therefore be used in the initial stages of reactor design. (c) 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.