The Essential Climate Variables (ECVs),such as the atmospheric thermodynamic state variables and greenhouse gases,play an important role in the atmosphere physical processes and global climate change.Given the need of improvements in existing ground-based and satellite observations to successfully deliver atmosphere and climate benchmark data and reduce data ambiguity,the Climate and Atmospheric Composition Exploring Satellites mission (CACES) was proposed and selected as a candidate mission of the Strategic Priority Research Program of Chinese Academy Science (SPRPCAS).This paper presents an overview of the key scientific questions and responses of ECVs in relation to global change;the principles,algorithms,and payloads of microwave occultation using centimeter and millimeter wave signals between low Earth orbit satellites (LEO-LEO microwave occultation,LMO) as well as of the LEO-LEO infrared-laser occultation (LIO);the CACES mission with its scientific objectives,mission concept,spacecraft and instrumentation.