As a result of investigations showing that communication problems can be a significant contributing factor to major aviation accidents, the International Civil Aviation Organization (ICAO) has established a set of Language Proficiency Requirements. All pilots and air traffic controllers engaged in international aviation must be certified by their national civil aviation authorities as meeting the requirements by March 2011. This has created a demand for tests designed to assess the speaking and listening skills of aviation personnel, and language testers have become involved as developers and evaluators of the various assessment instruments. The present paper gives an overview of the issues and introduces the themes discussed by the other contributors to this special issue of the journal, covering both the linguistic nature of aviation communication and more practical considerations in test design.