In this chapter we address the conundrum of the surprisingly long runout of large rock avalanches, which has been the subject of many investigations since it was first recognized in the nineteenth century in Switzerland. After describing the nature of the problem quantitatively, we briefly outline the many explanations that have been put forward to explain it; we also describe the wide variety of circumstances in which long runout is known to occur. We then examine the ability of the proposed explanations to apply to this range of circumstances, in order to identify those explanations that can work in all the environments in which long runout occurs. The process of dynamic rock fragmentation appears to be the only mechanism that satisfies this criterion. We outline this mechanism and its energetics in more detail, and summarize its recent success in quantitatively explaining the 40 km runout and the morphological characteristics of the 25 km3 Socompa debris avalanche deposit in Chile.