A dynamic mixer comprising two mixing parts which are rotatable relative to each other about a predetermined axis of rotation, each of said mixing parts having a mixing face, between which is defined a flow path which extends between an inlet and an outlet, each of said mixing faces comprising a series of annular steps centered on the predetermined axis of rotation, having a plurality of offset and overlapping cavities formed therein, such that material moving between the mixing faces of the two mixing parts from the inlet to the outlet is transferable between overlapping cavities.