A Pension fund is a pool of assets forming an independent legal entity that are bought with the contributions to a pension plan for the exclusive purpose of financing pension plan benefits.Pension funds are important shareholders of listed and private companies. They are especially important to the stock market where large institutional investors like the Ontario Teachers' Pension Plan dominate.

The largest 300 pension funds collectively hold about $6 trillion in assets. In January 2008, The Economist reported that Morgan Stanley estimates that pension funds world-wide hold over US$20 trillion in assets, the largest for any category of investor ahead of mutual funds, insurance companies, currency reserves, sovereign-wealth funds, hedge funds, or private equity.