Using weekly call option prices on twenty-five stocks over a ten-year period (1975–1985) and calls on the S&P 500 stock-index futures, we find that ex ante market volatility follows a mixed mean-reverting diffusion with noise process. Changes in volatility are correlated across stocks and a marketwide volatility effect is found. Strong forces pull the volatility back to its long-term value. The findings suggest the development of new option pricing models.