Citadel Investment Group is, as of September 2007, a $16.5 billion hedge fund based in Chicago, Illinois, founded by billionaire trader Kenneth C. Griffin. It is one of the world's largest hedge funds. The firm's daily trading volume amounts to approximately 3% of average daily trading activity in London, New York and Tokyo.The fund requires longer "lock-up" periods from investors than many other hedge funds. Though Citadel employs over 1,000 individuals globally, its flagship operation is located in Citadel Center, a $355 million office tower in downtown Chicago (in 2006 the tower was purchased for $560 million by Robert Gans).. Citadel also has offices in New York, Tokyo, Hong Kong, San Francisco, and London. Of the 100 largest hedge funds, only Citadel is based in Chicago ^. Citadel is the eleventh largest hedge fund in the world. it is also the second largest multi-strategy hedge fund in the world.

Despite prior talk in Wall Street that Citadel was considering an IPO and Kenneth C. Griffin mentioning that possibility in an interview, in April 2006 a spokesman for Citadel said the firm currently has no such plans. However, in November 2006 Citadel became the first hedge fund to issue bonds. In a bond offering led by Lehman Brothers and Goldman Sachs, Citadel announced it would sell $2 billion worth of notes.The bonds have been given an investment-grade rating by Standard & Poor's.

Citadel reportedly has a large stake in Google, Conor Medysystems, Alexion Pharmaceuticals, FirstFed Financial Corporation , uranium (an $11 million stake in Cameco), Dow Jones & Co., KB Home, Enzon Pharmaceuticals, K-V Pharmaceutical Company , the London Stock Exchange, and Banta Corporation . In November, Citadel invested $100 million in Xcel Energy; in November 2006, Citadel agreed to invest $28 million in mining company China Shen Zhou .

In 2004, Citadel founded CIG Re, a Bermuda-based catastrophe reinsurer. In 2005, the hedge fund founded a $500 million catastrophe reinsurer in Bermuda called New Castle Re.

Citadel also has multiple subsidiaries such as Kensington Global Strategies (Citadel's largest fund), Wellington Partners  (Citadel's oldest fund and its flagship fund), Citadel Equity Fund, Citadel Finance, and Citadel Derivatives Group, which controls 10% of the Philadelphia Stock Exchange . In 2000, Citadel's Wellington affiliate achieved a 52.6% return. Since January 2005, Citadel Derivatives Group has been a Lead Market Maker on the trading floor of the Pacific Coast Exchange . In a strategic partnership with about ten other financial institutions, Citadel Derivatives Group is also a joint owner of the International Securities Exchange 

In 2006, Citadel and JP Morgan Chase acquired the energy porfolio of failed hedge fund Amaranth Advisors, which had suffered a 65% ($6 billion) loss in assets 

In 2007, Citadel acquired a sizable stake in online brokerage

Citadel's daily trading activity reportedly accounts for "more than 10% of daily U.S. listed equity options contract volume" 

View detail: Citadel Investment Group