This paper presents recent developments and state of the art for transcritical CO2 cycle technology in various refrigeration, air-conditioning and heat pump applications. The focus will be on fundamental process and system design issues, including discussions of properties and characteristics of CO2, cycle fundamentals, methods of high-side pressure control, thermodynamic losses, cycle modifications, component/system design, safety factors, and promising application areas. The article provides a critical review of literature, and discusses important trends and characteristics in the development Of CO2 technology in refrigeration, air-conditioning and heat pump applications. Advanced cycle design options are also introduced suggesting possible performance improvements of the basic cycle. (C) 2003 Published by Elsevier Ltd.