The fundamental understanding of the mixing process is essential for scale-up in chemical development. Mixing is the reduction or elimination of inhomogeneity of phases that are either miscible or immiscible. The goal is to either reduce or eliminate temperature or concentration gradients, or to ensure good dispersion of multiple phases. Good mixing is desirable for several reasons, including side-reactions or byproduct formation, improving mass transfer in multi-phase systems, or ensuring fast heat transfer. Mixing efficiency is influenced by the type of material to be mixed, the design of the stirrer and the reactor, the mixing regime, but also the position of the feed tube and the operating conditions. [Handbook of Industrial Mixing, Science and Practice. Paul, E. Wiley (2004)]. Chemical reactions in a stirred tank, where the reagents may be present in more than one phase (liquid, gaseous, or solid), require intensive interfacial contacting to facilitate optimal mass transfer. Improper or poor mixing may result in a low reaction rate, low yield, poor selectivity, or increased concentration of impurities, which increase the costs of manufacturing significantly. Mass transfer and kinetics can compete and contribute to the overall reaction rate. Process scale-up and optimization require that the impact of mixing on the reaction rate be quantified. Laboratory reactors must be operated under conditions that will allow meaningful process characterization and scale-up.