A shredder, comprising: a first knife assembly (120; 220) comprising a plurality of first rotatable knives that are adjacently arranged in a first direction; an elongated sieve (130; 230) extending in the first direction and arranged spaced apart from the first knife assembly and facing a first side of the first knife assembly; and a drive system for driving the plurality of first knives; wherein the first knife assembly is configured to cut, punch, tear or otherwise deform pieces to be shredded fed to a second side of the first knife assembly into smaller pieces, and to push the smaller pieces through openings (131, 131_1, 131_2, 131_3, 131_4) of the sieve; characterized in that the shredder further comprises a guiding system including a hollow rotatable guiding unit elongated in the first direction from a first end toward an opposing second end, said guiding unit being configured to rotate about a rotational axis that is substantially parallel to the first direction; wherein the first knife assembly is arranged inside the guiding unit, and wherein the drive system is further configured for driving the rotatable guiding unit; wherein the guiding unit is configured to collect pieces that are pushed through the openings of the sieve and to provide the collected pieces to the second side of the first knife assembly for subsequent cutting, punching, tearing or otherwise deforming; wherein the guiding system is configured to displace pieces that have been cut, punched, torn or otherwise deformed in the first direction before these pieces are provided to the second side of the first knife assembly for said subsequent cutting, punching, tearing or otherwise deforming; and wherein a size of the openings of the sieve and/or a thickness of the first knifes decreases in the first direction. a first knife assembly (120; 220) comprising a plurality of first rotatable knives that are adjacently arranged in a first direction; an elongated sieve (130; 230) extending in the first direction and arranged spaced apart from the first knife assembly and facing a first side of the first knife assembly; and a drive system for driving the plurality of first knives; wherein the first knife assembly is configured to cut, punch, tear or otherwise deform pieces to be shredded fed to a second side of the first knife assembly into smaller pieces, and to push the smaller pieces through openings (131, 131_1, 131_2, 131_3, 131_4) of the sieve; characterized in that the shredder further comprises a guiding system including a hollow rotatable guiding unit elongated in the first direction from a first end toward an opposing second end, said guiding unit being configured to rotate about a rotational axis that is substantially parallel to the first direction; wherein the first knife assembly is arranged inside the guiding unit, and wherein the drive system is further configured for driving the rotatable guiding unit; wherein the guiding unit is configured to collect pieces that are pushed through the openings of the sieve and to provide the collected pieces to the second side of the first knife assembly for subsequent cutting, punching, tearing or otherwise deforming; wherein the guiding system is configured to displace pieces that have been cut, punched, torn or otherwise deformed in the first direction before these pieces are provided to the second side of the first knife assembly for said subsequent cutting, punching, tearing or otherwise deforming; and wherein a size of the openings of the sieve and/or a thickness of the first knifes decreases in the first direction.