Abstract: A multi leaf collimator for a radiation beam the particles of which is selected from the group consisting of high energy photons, electrons, protons and heavy ions which are emitted from a small effective radiation source. The collimator comprises low hight, elongated, curved, in cross section wedge-shaped leaves arranged side by side in opposed pairs. The configuration is such that the parts of the surfaces intersecting the irradation field will always be directed generally towards the radiation source. A protective casing surrounding a frame on which the collimator is mounted can be filled with helium gas to optimize the collimator for use with electron and photon beams in the energy range from 1 up to 50 MeV. The individual settings of the leaves are supervised by a TV-camera which also can be used to set the leaves in predetermined positions.