A specialized technology for reasoning with descriptions that extends the class of useful inferences beyond simple inheritance has been developed as part of the Strategic Computing Initiative of the US Dept. of Defense's Defense Advanced Research Projects Agency. Loom and the Knowledge Representation Editing and Modeling Environment (KREME), which capture this technology in the form of an inference engine, called a classifier, are described. These systems share the notion that the ability to define and reason with descriptions is basic to the task of knowledge representation. Loom supports a description language (the frame component) and a rule language, and uses its classifier to help bridge the gap between the two. The classifier gives Loom the additional deductive power to provide inference capabilities not found in current knowledge-representation tools. KREME, a knowledge acquisition and editing tool, uses a classifier to help knowledge engineers maintain consistency while developing knowledge bases. One of KREME's functions is to bring both actual and potential conflicts to developers' attention. KREME can also help merge separately developed knowledge bases. Description unification is compared with Prolog-style unification, and the benefits of description technology are examined.