A deliberate mixing of particulate solids from different ingredients is often an essential operation, yet it is not a trivial exercise, and requires considerable efforts to achieve good mixing of particulate solids with different size and density. The key to do so relies on how to select proper mixers based on mechanisms of powder mixing, and then on how to design the right procedure accordingly. The mechanisms of powder mixing are roughly identified as: (1) shear mixing; (2) diffusive mixing; and (3) convective mixing.