Chapters 19 and 20 of this book, on instability around underground excavations, are direct analogues of Chapters 17 and 18, which were concerned with surface excavation instability. In this chapter, the underground instability mechanisms are presented. In Chapter 20, the design and analysis of underground excavations are discussed in the context of these mechanisms. In Section 16.2, the distinction between structurally-controlled and stresscontrolled instability mechanisms is explained. Accordingly, this chapter has been devoted to these two primary instability modes, considering also composite instability modes and the effect of time and weathering on excavation stability.