At the time of the advent of the electron probe microanalyzer, this instrument was considered primarily a tool for metallurgical research (l,2). Since then the domain of its application has been extended to several other fields, although metallurgy is still its main area of use. In a recent bibliographic review (3) 76 references to metallurgical applications are listed, while only 26 publications mentioned refer to mineralogical and biological applications. In his doctoral thesis in 1951 (2) Castaing outlined its use for intermetallic diffusion, microsegregation, study of inclusions, and lattice parameter measurements by the Kossel line technique. While the analytical technique then proposed by him has remained essentially unchanged, technical improvements and implementations of the instrument have widened its field of application. Extension of the spectral range to 10 A has increased the number of elements amenable to electron probe analysis, adding the important group from chlorine to magnesium. The possibility of further advance to incorporate oxygen, nitrogen, and carbon (4) is probably the most exciting prospect for the near future. Another important innovation is the creation of the scanning electron microprobe (S) that combines the features of the static electron probe and those of the scanning electron microscope proposed by von Ardenne (6) and further developed in England (7). The advantages of electron beam scanning for metallurgical applications were conclusively shown by Melford and Duncumb (8), and a similar device based on mechanical movement of the specimen was recently developed in France (9). Equally relevant is the increasing availability of highly sophisticated and efficient commercially built instruments now manufactured in the United States, France, England, and Japan. Consequently the number of microprobes used in metallurgical establishments is increasing at an impressive rate.