It was proposed in the 1950's that X-rays could be used to probe magnetization densities. After some experimental demonstrations of the technique, applications have shown that tle use of magnetic X-ray scattering c o m -b i n e d w i t h s y n c h r o t r o n r a d i a t i o n c a n l e a d t o i m p o r t a n t n e w d i s c o v e r i e s i n the magnetism of solid materials. Several important features have emerged, especially the resonant magnetic X-ray scattering: large enhancements of scattered intensities, site and species selectivity, X-ray energy and polarization dependence. These properties make it possible to study new phenomena: details of magnetic structures of materials including micro-crystals, new aspects of magnetic plase transitions and surface magnetism.