Volume 6 of Developments in Applied Spectroscopy presents a collection of twenty-eight selected papers from those that were pre sented at the Eighteenth Mid-America Symposium on Spectroscopy held in Chicago, May 15 to 18, 1967. In general, the papers selected by the editors are those of the symposium type and not those papers pertaining to a specific research topic that one expects to be sub mitted to a journal. Not all of the submitted papers were included. Some revisions could not meet the deadline and others were not accepted based on the advice of the reviewers. It is the opinion of the committee that this type of publication has ·an important place in the literature. The Mid-America Symposium is sponsored annually by the Chicago Section in cooperation with the Cincinnati, Detroit, Indianapolis, Milwaukee, Niagara Frontier, and St. Louis Sections of the Society of Applied Spectroscopy, and the Chicago Gas Chromatography Group. Although the Mid-America is often thought of as a regional meeting, its attendees and authors generally come from all parts of the United States and Canada. Both applied and theoretical principles were provided in sessions on X-ray, emission, atomic-absorption, nuclear magnetic resonance, infrared, Raman, nuclear-particle, and gamma ray spectroscopy; activation analysis; and gas chromatography. In addition, there were symposia on absorption spectra of biologically significant molecules; the structure of ice, water, and aqueous solu tions; air and water pollution analyses; and the practical application of statistics.