Aon Corp. is a global provider of insurance brokerage services, insurance products, and risk and insurance advice, web based risk management information systems, as well as other consulting services. The company operates in three major segments: commercial brokerage, consulting services, and consumer insurance underwriting. Aon Corp.'s brokerage unit, Aon Risk Services, provides retail property/casualty, liability, and other insurance products for groups and businesses, as well as risk management services. Aon Benfield handles reinsurance brokerage services for aviation, marine, energy, professional liability, and other niche and specialty business lines. Its consulting unit, Aon Consulting Worldwide, specializes in employee benefits administration. The risk and insurance brokerage segment accounted for 82% of total revenue from continuing operations in 2007, and the consulting segment accounted for 18%.

The company employs approximately 59,000 workers in its 500 offices in 120 countries. Aon is the world's second largest insurance brokerage, and largest reinsurance brokerage.

Aon was created in 1982, when the Ryan Insurance Group (founded by Pat Ryan in the 1960s) merged with the Combined Insurance Company of America (founded by W. Clement Stone in 1919). In 1987, that company was introduced to Wall Street as Aon, a Gaelic word meaning “oneness”. Combined Insurance was sold to ACE Limited in April 2008.

On August 22, 2008, Aon announced that it had acquired London-based Benfield Group. The acquiring price was US$1.75 billion or £935 million, with US$170 million of debt. Today, the company is best known internationally as the principal sponsor of English football giant, Manchester United.

On July 12, 2010, Aon announced that it has agreed to buy Lincolnshire, IL based Hewitt Associates for $4.9 billion in cash and stock.

Aon built a global presence through purchases. In 1997 it bought The Minet Group, as well as insurance brokerage Alexander & Alexander Services, Inc. in a deal that made Aon (temporarily) the largest insurance broker worldwide. The firm made no U.S. buys in 1998, but doubled its employee base with purchases including Spain's largest retail insurance broker, Gil y Carvajal, and the formation of Aon Korea, the first non-Korean firm of its kind to be licensed there.

Manchester United sponsorship

On June 3, 2009, Aon signed a four year shirt sponsorship deal with English football giant Manchester United. On June 1, 2010, Aon replaced troubled American insurance company AIG as the principal sponsor of the club. The Aon logo is prominently displayed on the front of the club's new shirts.

The deal is said to be worth £80 million over four years, replacing United's deal with AIG as the most lucrative shirt deal in history at the time.