This paper documents the history of aggregate positions in US index options and investigates the driving factors behind use of this class of derivatives. We construct several measures of the magnitude of the market and characterize their level, trend, and covariates. Measured in terms of volatility exposure, the market is economically small, but it embeds a significant latent exposure to large price changes. Out-of-the-money puts are the dominant component of open positions. Variation in options