This conclusion presents some closing thoughts on the key concepts discussed in the preceding chapters of this book. The book examines both the need for a common language, or lingua franca, for the specific purpose of air traffic controller-pilot communications, and the difficulties associated with achieving that goal. It discusses the challenges associated with implementing the International Civil Aviation Organization (ICAO) language proficiency requirements (LPR) in relation to language testing. The book discusses the impact of characteristics of air traffic controller messages and pilot cognitive workload on communication and performance accuracy. It explores various facets of the concept of Aviation English (AE) and the role of Aviation English in aviation communication. The book shows that AE is a restricted variety of English designed and used for the specific purpose of communication in the aviation environment.