In finance, volatility arbitrage (or vol arb) is a term for financial arbitrage techniques directly dependent and based on volatility. A common type of vol arb is type of statistical arbitrage that is implemented by trading a delta neutral portfolio of an option and its underlying. The objective is to take advantage of differences between the implied volatility of the option, and a forecast of future realized volatility of the option's underlying. In volatility arbitrage, volatility rather than price is used as the unit of relative measure, i.e. traders attempt to buy volatility when it is low and sell volatility when it is high. Overview To an option trader engaging in volatility arbitrage, an option contract is a way to speculate in the volatility of the underlying rather than a directional bet on the underlying's price. If a trader buys options as part of a delta-neutral portfolio, he is said to be long volatility. If he sells options, he is said to be short volatility. So long as the trading is done delta-neutral, buying an option is a bet that the underlying's future realized volatility will be high, while selling an option is a bet that future realized volatility will be low. Because of the put–call parity, it doesn't matter if the options traded are calls or puts. This is true because put-call parity posits a risk neutral equivalence relationship between a call, a put and some amount of the underlying. Therefore, being long a delta-hedged call results in the same returns as being long a delta-hedged put. Volatility arbitrage is not "true economic arbitrage" (in the sense of a risk-free profit opportunity). It relies on predicting the future direction of implied volatility. Even portfolio based volatility arbitrage approaches which seek to "diversify" volatility risk can experience "black swan" events when changes in implied volatility are correlated across multiple securities and even markets. Long Term Capital Management used a volatility arbitrage approach. Forecast volatility To engage in volatility arbitrage, a trader must first forecast the underlying's future realized volatility. This is typically done by computing the historical daily returns for the underlying for a given past sample such as 252 days (the typical number of trading days in a year for the US stock market). The trader may also use other factors, such as whether the period was unusually volatile, or if there are going to be unusual events in