Abstract: An apparatus for reducing the size of, and/or for mixing, solid, pasty and/or liquid material. The apparatus includes a cylindrical housing in which is disposed at least one grinding roller, which cooperates with the inner surface of the housing. Each grinding roller is mounted between two bearing disks in such a way as to be rotatable about its central axis and eccentrically relative to the central axis of the housing. The bearing disks in turn are rotatably driven about the central axis of the housing. In order to enable the grinding rollers to execute the necessary deflection movements, and to avoid over stressing the bearings or supports for the grinding rollers, both end faces of each grinding roller are provided with an axle end that is guided, with play, between two drive wheels that are rotatably mounted on the bearing disks. When the bearing disks rotate, the drive wheels rotatably drive the grinding rollers, which are pressed by centrifugal force against the inner surface of the housing.