Over two decades, ETFs have become one of the most popular investment vehicles among retail and professional investors due to their low transaction costs and high liquidity, taking market share from traditional investment vehicles such as mutual funds and index futures. Research has shown that in addition to the benefits of enhanced price discovery, ETFs add noise to the market: Prices of underlying securities have higher volatility, greater price reversals, and higher correlation with the index. Arbitrage activity is a necessary component in minimizing the price discrepancy between ETFs and the underlying securities. During turbulent market episodes, however, arbitrage is limited and ETF prices diverge from those of the underlying securities. Expected final online publication date for the Annual Review of Financial Economics Volume 9 is Novenber 1, 2017. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.