In this chapter, the basic principles of diffusers will be developed. Following the same style of Chapter 2, this chapter will be driven by application. In many respects, the right and wrong places to use diffusers are still being worked out.1,2 There are locations, such as rear walls of large auditoria used for classical music, where there is a broad consensus that diffusers are a good treatment to prevent echoes and are better than absorbers. The case for using diffusers in some other places is less clear-cut, and until further research is undertaken, some applications are going to be based more on precedence and intuition rather than scientific fact. Having said this, much has been learned in recent decades, which can help to ensure that diffusers are used where they are needed.