A cutting unit for comminuting bulky feedstock and a device into which such a cutting unit is integrated. The cutting unit has a cutting chamber, which is arranged along a conveying direction and is formed by paired opposite cross walls and longitudinal walls. A knife for comminuting the feedstock can be moved transverse to the conveying direction across the entire cross section of the cutting chamber from one cross wall to the opposite cross wall. To also assure the complete cutting through of fiber-containing or film-shaped feedstock, a counter knife is arranged in the cross wall opposite to the blade of the knife which is stationary relative to the cutting chamber and which during the cutting process forms a cutting gap with the blade of the knife. The wall segment of the opposite cross wall is placed movable in the advance direction of the knife.