Publisher Summary This chapter reviews the various types of impellers, the flow patterns generated by these agitators, correlation of the dimensionless parameters (i.e., Reynolds number, Froude number, and Power number), scale-up of mixers, heat transfer coefficients of jacketed agitated vessels, and the time required for heating or cooling these vessels. Various types of reactors are increasingly employed in the mixing of reactants to achieve the required yield at a given rate. Molecular diffusion is essential to achieve mixing between two liquids and complete the mixing process. If the fluids being mixed are gases, the rate of molecular diffusion is very high and thus the time required is extremely small. In the case of liquids where the rate of molecular diffusion is slow, knowledge of the turbulence is required so as to estimate the size of the smallest eddy and establish the time for molecular diffusion. A set of criteria provides a measure of the level of mixing. The scale of segregation measures the size of the unmixed clumps of pure components; the intensity of segregation describes the effect of molecular diffusion on the mixing process. It is defined as the difference in concentration between the neighboring clumps of fluid.