In studies requiring determinations of isotopic and/or
trace-element abundances with very high spatial resolution, the
ion microprobe remains the premier tool for the geochemist and
cosmochemist. In SIMS analysis, a focused beam of ``primary'' ions
(a few 
m in diameter) bombards a solid sample in order to
obtain a localized analysis. The bombardment or ``sputtering''
removes atoms from the polished surface of the specimen, some of
which are ionized and can be accelerated into the entrance slit of
a mass spectrometer [see Reed, 1989]. Both custom-built and
commercial ion microprobes have been employed in geo/cosmochemical
research for approximately 15 years.
The ion microprobe's capabilities for in situ quantification for many elements at sub-ppm concentrations, including those with Z<11, are complementary to, and provide a natural extension of, the kind of analytical data obtained via the electron microprobe. Many applications have involved REE abundances and experimental measurement techniques in this field have become rather mature [see MacRae et al., 1993 and references therein]. The present quadrennium has seen a continuation of instrumental and technique developments, particularly in the design of new high-sensitivity ion microprobes and in the area of stable isotope measurements (e.g., VG ISOLAB, SHRIMP II, and CAMECA ims 1270). There has also been a gradual shift in emphasis toward applications. ``Experimental techniques'' sections, which formerly constituted a large part of geo/cosmochemical papers that utilized SIMS analyses, are now often rather terse. Ion microprobes are in routine use for U-Pb zircon dating, measurements of trace element abundances, and stable isotope measurements---especially in interstellar dust, but increasingly also in terrestrial igneous and metamorphic rocks. Given the large number of papers published in the last 4 years that contain ion microprobe data from labs worldwide, the reader is referred to Ireland [1994] for a global SIMS review.