With this review, we alter the approach from either the 1972 and 1974 reviews which concentrated exclusively on significant advances or the pre-1972 reviews which enumerated everything that had occurred in the field. This time we try to summarize the "state of the art" and point out where progress has been made or may be made eventually.In particular, the current status of instrumentation, applications, and data interpretation are described, and the various approaches are compared. INSTRUMENTATIONThis section of the review is divided into two categories, namely, the commercial instrumentation for routine use and the specialized instrumentation under development.Routine Instrumentation. There are three classes of routine instruments for x-ray fluorescence analysis: a) The sequential wavelength-dispersion spectrometer in which the crystal and detector are scanned through the wavelength range and measure one element at a time; b) The multiple-crystal simultaneous wavelength dispersion instrument in which a fixed crystal and detector is set up for each of several elements (up to 24 elements in some commercial machines); c) The energy-dispersion instrument in 70, 57 (1974).

To reiterate the point of view expressed in the 1972 review, it is not our intent merely to report everything that has happened; neither is it our intent to itemize every instance of progress (ideally all publications should represent progress but unfortunately they do not because of hasty submission of partly finished work (1) or careless review of manuscripts). Rather, the intent here is to select areas of significant advances in X-ray theory or practice and to attempt to show what these advances consist of. It is our opinion that the significant advances of the past two years have come about by adding together small items of progress rather than by spectacular quantum jumps in instrumentation or techniques.Again to reiterate the caveat lector of the 1972 review, the selection of topics for discussion is unquestionally subjective to some extent, and only future progress will demonstrate whether or not the items covered herein are the most significant. HISTORICAL NOTEA most fascinating look backward at the early history of X-ray spectroscopy is provided by the book, "Moseley and the Numbering of the Elements" by Jaffe (2). The book reads almost like a novel as it traces the story of the people as well as the interlocking events that took place in atomic theory in the early 1900's. It makes the reader appreciate the experimental ingenuity as well as the searching intellects which prevailed during those challenging years following the discovery of X-rays in 1895 until Moseley's experiments in 1913. Contributions by giants such as J.