The technique of X-ray magnetic circular dichroism (XMCD) is presented from an experimental perspective. Absorption measurements in a net magnetic field can be performed using circularly polarized X-rays produced by bending magnets or insertion devices at the synchrotron and by employing detection methods such as X-ray transmission, total electron yield or fluorescence yield. XMCD is manifest in absorption edges of open-shell atoms, such as the K, L_2,3 and M_2,3 edges of transition metals, the L_2,3, M_4,5 and N_4,5 edges of rare earths and the M_4,5, N_4,5 and O_4,5 edges of actinides. The profoundly different characteristic line shapes provide a fingerprint for the spin and orbital character of the valence shell. The integrated intensities over the dichroic absorption edges can be related via sum rules to the expectation values of the spin and orbital magnetic moment in the ground state. The application of these sum rules has transformed XMCD into a powerful technique. This element-, site- and symmetry-specific technique enables a broad range of novel and exciting studies on the electronic and magnetic structure of modern materials.