This report traces the evolution of aircraft icing severity definitions and of the Federal Aviation Administration (FAA) regulations governing flight in icing conditions in order to understand the intent of each and how they relate to each other. There have been several changes in both the definitions and the regulations over time, and part of the problem is that the definitions have not been updated or clarified to account for current regulations. Much confusion has resulted and, in order to improve the situation, new and updated definitions have been recently proposed by a new working group established as part of the 1997 FAA In-Flight Icing Plan.