Despite their strong positive average returns across numerous asset classes, momentum strategies can experience infrequent and persistent strings of negative returns.These momentum crashes are partly forecastable.They occur in "panic" states -following market declines and when market volatility is high -and are contemporaneous with market rebounds.We show that the low ex-ante expected returns in panic states are consistent with a conditionally high premium attached to the option-like payoffs of past losers.An implementable dynamic momentum strategy based on forecasts of momentum's mean and variance approximately doubles the alpha and Sharpe Ratio of a static momentum strategy, and is not explained by other factors.These results are robust across multiple time periods, international equity markets, and other asset classes.