In this chapter, we return to the topic of cache coherence that we introduced in Chapter 2. We defined coherence in Chapter 2, in order to understand coherence’s role in supporting consistency, but we did not delve into how specific coherence protocols work or how they are implemented. This chapter discusses coherence protocols in general, before we move on to specific classes of protocols in the next two chapters. We start in Section 6.1 by presenting the big picture of how coherence protocols work, and then show how to specify protocols in Section 6.2. We present one simple, concrete example of a coherence protocol in Section 6.3 and explore the protocol design space in Section 6.4.