Hydrodynamics in a mixing tanks results from the interaction between the tank geometry and the performance characteristics of the tank internals such as impeller and draft tube. In this book, the standard stirred tank configurations are investigated using both laser Doppler velocimetry (LDV) and computational fluid dynamics (CFD) techniques. The work first focuses on establishing the velocity field in a liquid-only system and then progresses to a solid-liquid system. In the liquid only system, it is shown that a draft tube can be used in a tank stirred by the Rushton turbine, and this improves mixing efficiency and reduces power consumption. For the solid-liquid systems, CFD simulation techniques are developed to determine multiphase mixing features such as cloud height and off-bottom suspension. Poly-disperse multiphase simulation results show the effect of particle size distribution and that of the drag model. The results provide useful information for process optimization and scale up.