This chapter is all about mass extinctions. I have talked about them previously but here I provide much more detail. I introduce the Gubbio section in Italy where a complete end – Cretaceous record allowed Louis and Walter Alvarez to recognised in possibility that a meteor played a significant role in triggering the end-Cretaceous mass extinction. I provide a definition of what constitutes a mass extinction, talk about how many there are in the fossil record and explain how we know how many species go extinct at each of the big five mass extinctions. Scientists have always been fascinated by the possibility of there being a cyclic pattern of mass extinctions. After reviewing the data and spending a little time the Nemesis model of cyclicity I suggest that this is not the case. Mass extinctions are associated with major shifts in the Earth System. Reflecting on this linkage, the chapter ends with what amounts to a general model for all mass extinctions.