Thesis: Ph. D., Massachusetts Institute of Technology, Sloan School of Management, September, 2020Cataloged from student-submitted PDF version of thesis.Includes bibliographical references.Chapter 1 constructs a theoretical model of an ETF. Conventional wisdom warns that exchange-traded funds (ETFs) harm stock price discovery, either by ``stealing'' single-stock liquidity or forcing stock prices to co-move. Contra this belief, I develop a theoretical model that investors with stock-specific information trade both single stocks and ETFs. While the ETF is payoff-redundant, asymmetric information and a position limit for informed traders combine to make the ETF non-redundant. Single-stock investors can access ETF liquidity by means of this tan...