Description: “What is the likelihood of market crashes? This article seeks to provide an answer to this and related questions by focusing mainly on the crash of October 1987. Specifically, we shall seek answers to three questions: 1. Given the history of market returns, was the crash of ‘87 unusual? 2. How do conditional variance models such as GARCH behave around periods of extreme moves in the market? 3. What is the impact of the crash on backtesting and performance evaluation?” Editor’s Note: The following article originally appeared in the Winter 1998 issue of The BARRA Newsletter, Horizon, and is reprinted here with permission.