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Relativistic electrons subjected to periodic acceleration in an undulator (planar or helical) emit quasi-monochromatic synchrotron radiation, which is tunable and polarized. Such emissions have been observed previously in e(-) beams accelerated in linear accelerators or synchrotrons. Synchrotron radiation laboratories are developing undulators, which will constitute exceptional photon sources when placed on storage rings: their brightness is expected to be several orders of magnitude greater than the brightness of the same e(-) beam in normal bending magnets, and, depending upon the e(-) energy, they can be tuned from the visible to the x-ray range. Recent experiments on storage rings show that the observed emission agrees with theoretical predictions.