Based on a comprehensive understanding and grasp of carbon emission reduction projects, this book proposes the concept of carbon emission reduction system engineering and establishes related theories, technologies, and practical systems of carbon emission reduction system engineering. The concept of carbon emission reduction system engineering includes two basic connotations. One is to clarify that carbon emission reduction engineering is a new type of complex system engineering, different from traditional engineering; the other is to clarify that we should start from the perspective of complex system engineering to carry out research on carbon emission reduction theoretical methods and apply them to related carbon emission reduction engineering practices. This chapter will propose a carbon emission reduction system engineering system based on a clear definition and characteristics of carbon emission reduction system engineering, summarize the five core issues facing carbon emission reduction, such as how much to reduce, when to reduce, who to reduce, how to reduce, and what effect, and refine the short-term and long-term, local and overall, government regulation and market mechanism, development and emission reduction “four pairs” of coordinated problems, and finally outline the carbon emission reduction “time–space–effect–benefit” coordinated theory, technology, and practical system to be introduced in the whole book.