A multi-leaf collimator for use in a radiotherapeutic apparatus comprises a plurality of elongate narrow leaves arranged side-by side and supported in a frame, the frame having upper and lower formations for guiding each leaf into which extend ridges on the upper and lower edges of the leaves, thereby to allow the leaves to move in a longitudinal direction, the upper and lower formations being aligned so that the sides of the leaves when fitted are at a non-zero angle to the beam direction, the upper and lower ridges being located on the upper and lower edges of the leaves so that a line joining their centres is at a non-zero angle to the sides of the leaf, tilted relative to the sides in a sense opposite to that of the beam. An outer face of the upper and/or lower ridges can be aligned with a side face of the leaf, for ease of manufacture. A radiotherapeutic apparatus is also disclosed, comprising a source of radiation and a multi-leaf collimator for shaping the radiation emitted by the source, the multi-leaf collimator being as set out above.