A dynamic mixer (10; 30; 50; 70; 90; 120; 150) 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 (16; 36; 56; 76; 96; 126; 156) for material to be mixed and an outlet (17; 37; 57; 77; 97; 127; 157), each of said mixing faces comprising a series of annular steps (18, 20; 38, 40; 58, 60; 78, 80; 98, 100; 128, 130; 158, 160) centred on the predetermined axis of rotation, having a plurality of cavities (23, 24; 43, 44; 63, 64; 83, 84; 103, 104; 133, 134; 163, 164) formed therein, said cavities defining flow passages bridging adjacent steps on each of the two mixing parts, each of said mixing faces being mutually positionable such that the steps of one mixing part extend towards recesses (19, 21; 39, 41; 59, 61; 79, 81; 99, 101; 129, 131; 159, 161) formed between the steps of the other mixing part, whereby cavities present in one mixing face are offset relative to, and overlap with, cavities present in the other mixing face in an axial direction or a transverse direction, such that material moving between the mixing faces of the two mixing parts from the inlet to the outlet is transferrable between overlapping cavities, wherein at least one step of one of the mixing parts and at least one adjacent step of the other of the mixing parts extends further in the axial direction than in a transverse direction, or vice versa, characterized in that at least one annular mixing zone (25; 45; 65; 85; 105; 135; 165) of substantially uniform volume is provided in between non-overlapping cavities of the two mixing parts. 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 (16; 36; 56; 76; 96; 126; 156) for material to be mixed and an outlet (17; 37; 57; 77; 97; 127; 157), each of said mixing faces comprising a series of annular steps (18, 20; 38, 40; 58, 60; 78, 80; 98, 100; 128, 130; 158, 160) centred on the predetermined axis of rotation, having a plurality of cavities (23, 24; 43, 44; 63, 64; 83, 84; 103, 104; 133, 134; 163, 164) formed therein, said cavities defining flow passages bridging adjacent steps on each of the two mixing parts, each of said mixing faces being mutually positionable such that the steps of one mixing part extend towards recesses (19, 21; 39, 41; 59, 61; 79, 81; 99, 101; 129, 131; 159, 161) formed between the steps of the other mixing part, whereby cavities present in one mixing face are offset relative to, and overlap with, cavities present in the other mixing face in an axial direction or a transverse direction, such that material moving between the mixing faces of the two mixing parts from the inlet to the outlet is transferrable between overlapping cavities, wherein at least one step of one of the mixing parts and at least one adjacent step of the other of the mixing parts extends further in the axial direction than in a transverse direction, or vice versa, characterized in that at least one annular mixing zone (25; 45; 65; 85; 105; 135; 165) of substantially uniform volume is provided in between non-overlapping cavities of the two mixing parts.