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I t is evident that light is perhaps the most important tool by which we know the world around us. This is true not only for our everyday experience, but also for the scientific pursuit of an understanding of nature. Experiments using electromagnetic waves, in a range of wavelengths going well beyond the relatively small range of visible light, have played an important role in the development of modern science. Atomic and molecular spectroscopy, i.e. the study of the characteristic wavelengths emitted by matter in the gas phase, has been fundamental in establishing the laws of quantum mechanics and has given us important information on the composition of stars and planets. Einstein's brilliant 1905 interpretation of the photoelectric effect has opened the way to photoemission spectroscopy,which is still today one of the most important probes ofthe electronic structure ofsolids and solid surfaces, using DV light. Rontgen's discovery ofx-rays in 1895, and the subsequent 1911 demonstration ofx-ray diffraction by crystals, by von Laue and others, laid the foundations of crystallography, by which we can unravel the atomic structure of crystals. Every secondary school student is familiar with Watson and Crick's exploit of 1953 which identified the double helix structure of DNA from the x-ray diffraction work of Franklin and Wilkins, probably the most famous piece of crystallographic work ever. It is therefore hardly surprising that scientists have been eager to obtain the brightest light sources, in order to understand the atomic and electronic structure ofmatter better and better. In parts of the infrared, in the visible and in the near DV, the invention of the laser has provided an extraordinary tool,which has led to many exciting discoveries and applications. Over a much broader range, encompassing not only the infrared and the visible, but also the whole of the UV and the x-rays to wavelengths well below 0.05 nm, the tool of choice for many scientists is synchrotron radiation (or synchrotron light) i.e. the bright emission of highly collimated electromagnetic waves from electrons (or positrons) orbiting at ultrarelativistic energies in storage rings with diameters of tens to several hundreds of meters (generally called with the slightly improper term synchrotrons). In spite of their large dimensions and associated cost, there are some 50 or so storage rings around the world built and operated solely for the purpose ofproducing light, and more are under construction. Although synchrotron light is ever so popular today, the basic phenomena underlying this way of producing light are by no means very recent or novel. The electromagnetic theory of Maxwell, which very naturally contains aspects later formalised in the special theory of relativity, is all that is needed to predict the basics of synchrotron light. The radiation of accelerated charges, e.g of charged particles following a circular trajectory at constant speed, and subject therefore to centripetal acceleration, has a very different angular distribution depending on whether the speed of

The quest for brilliance: light sources from the third to the fourth generation