The literature published on powder mixing is reviewed and the conclusion drawn that the operation is misunderstood and the situation confused because insufficient attention has been paid to the properties of particles being mixed. As an aid to simplification, powders are classified as free-flowing powders where the particle motion is individual, and non-free-flowing powders where the mixing is more concerned with the shuffling and subsequent breaking down of large agglomerates. This thesis considers the mixing of free-flowing powders only.