Harry Markowitz is an American economist known for pioneering modern portfolio theory. He developed the concept of the efficient frontier, which shows the set of optimal portfolios that produce the highest expected return for a given level of risk or the lowest risk for a given level of expected return. Markowitz proposed using statistical tools like variance to quantify an investment's risk and using expected returns and correlations between assets to construct portfolios. His work revolutionized the theory and practice of asset management and laid the foundation for much of the subsequent development in financial economics.