ISI has developed a citation-based method to reveal the cognitive structure of science, and how specialties emerge, converge, and diverge over time. The method combines co-citation clustering with multidimensional scaling techniques to produce maps from the Science Citation Index that depict the structure of science at varying resolutions, from small subspecialties to entire fields. Historiographs are also generated that track changes in subspecialties / fields over time. The presentation will disc uss how the field of chemistry, on a macro level, has evolved from 1983-88. A microanalysis will illustate changes within a subspecialty, e.g., reaction kinetics, from 1974-88. Also, Citation Classics in chemistry, identified by citation frequency data, will be presented. Prospective uses of these data by science journal editors, policymakers and administrators will be explored.