The invention relates to a leaf (1) for a multi-leaf collimator (2) for delimiting a high-energy beam (3, 3', 3'') of an irradiation device, in particular for conformational radiation therapy. According to the invention, the multi-leaf collimator (2) comprises a plurality of opposing leaves (1), which can be brought into the beam path (3, 3', 3'') by means of drives (4), in such a way that the contour (5) of said path can be shaped in accordance with the volume to be irradiated. The need for beam-absorbent material (7) is reduced, as the leaf (1) essentially only comprises a beam-absorbent material (7) of an appropriate thickness (8) in a region (6) that is brought into the path (3, 3', 3'') of the high-energy beam (3) during the course of all possible positional displacements (9). The invention also relates to a corresponding multi-leaf collimator (2), a device (22) for delimiting beams, and an irradiation device.