A mutual fund is a professionally managed investment fund that pools money from many investors to purchase stocks, bonds, and other securities. There are three main types of mutual funds: open-end funds that must purchase back shares daily at net asset value, closed-end funds that trade on an exchange, and unit investment trusts that have a fixed portfolio. Mutual funds provide advantages like diversification, liquidity, and professional management, but also have disadvantages like fees and less control over taxes. The first mutual funds were established in the 1700s in Europe and gained popularity in the US in the 1920s. Today, the largest mutual fund companies are Vanguard, Fidelity, and American Funds.