Harry Markowitz is an American economist best known for his pioneering work in the field of finance. He introduced the concept of Modern Portfolio Theory in the 1950s, which emphasizes the importance of diversification in investment portfolios. His research demonstrated how investors can optimize their returns by carefully selecting a mix of assets that balance risk and reward. In 1990, Markowitz was awarded the Nobel Prize in Economic Sciences for his contributions to financial economics. His theories have significantly influenced investment strategies and the way financial markets operate, making him a key figure in the development of quantitative finance.