In Chapters 7 and 8, we have presented snooping and directory coherence protocols in the context of the simplest system models that were sufficient for explaining the fundamental issues of these protocols. In this chapter, we extend our presentation of coherence in several directions. In Section 9.1, we discuss the issues involved in designing coherence protocols for more sophisticated system models. In Section 9.2, we describe optimizations that apply to both snooping and directory protocols. In Section 9.3, we explain how to ensure that a coherence protocol remains live (i.e., avoids deadlock, livelock, and starvation). In Section 9.4, we present token coherence protocols [11], a recently developed class of protocols that subsumes both snooping and directory protocols. We conclude in Section 9.5 with a brief discussion of the future of coherence.