Volume 27 of Reviews in Mineralogy provides a background to the TEM as a mineralogical tool, to give an introduction to the principles underlying its operation, and to explore mineralogical applications and ways in which electron microscopy can augment our knowledge of mineral structures, chemistry, and origin. Much time will be devoted to mineralogical applications. It provides sufficient information to allow mineralogists and petrologists to have an informed understanding of the data produced by transmission electron microscopy and to have enough knowledge and experience to undertake initial studies on their own. The opening chapters cover the principles of electron microscopy and chemical analysis using the TEM; while the following chapters consider mineralogical, petrological, and geochemical applications and their implications, for both low- and high-temperature geological environments. The Mineralogical Society of America sponsored a short courses in conjunction with their annual meetings with the Geological Society of America, and this volume represents the proceedings of the eighteenth in the sequence. This TEM course was convened October 23-25, 1992, at Hueston Woods State Park, College Comer, Ohio.