1. Introduction

In the context of increasing attention to sustainable and green chemistry, a variety of efforts have been devoted to atom economical and environmentally friendly methods to construct complex molecules. The incorporation of sulfur dioxide into small molecules is an attractive and promising strategy since it introduces the sulfonyl moiety from simple and readily available sources. Developing sulfur dioxide insertion reactions for organic synthesis has become an active research area in the last few years, particularly owing to the development of innocuous and bench-stable sulfur dioxide surrogates. In this chapter, we will highlight the importance of sulfonyl groups (—SO₂—) in diverse fields, introduce the basic properties of sulfur dioxide, and classify the reaction types derived from sulfur dioxide. We will also present the main contents and emphases of this book, and wish to give a comprehensive understanding of sulfur dioxide insertion chemistry to the readers.