In the Introduction, we have briefly discussed in general X-ray fluorescence, both wavelength dispersive and energy dispersive to put radioisotope induced X-ray emission in perspective. After mentioning the major advances in radioisotope induced X-ray emission we selected two areas to discuss in detail. First, the insertion of a radioisotope directly into the specimen and secondly, the application of small biological samples for trace elements at the µg/g concentration levels.