Aon plc is a global professional services firm headquartered in London that provides risk, retirement and health consulting. Aon has approximately 500 offices worldwide, serving 120 countries with 50,000 employees.

In 2011, Aon was ranked as the largest insurance broker in the world based on revenue. Aon was the principal partner and global shirt sponsor of the Premier League team Manchester United F.C. from 2010 until 2014.

Aon was created in 1982 when the Ryan Insurance Group merged with the Combined Insurance Company of America. In 1987, that company was renamed Aon, a Gaelic word meaning [one].

In January 2012, Aon announced that its headquarters would be moved to London.

W. Clement Stone's mother bought a small Detroit insurance agency, and in 1918 brought her son into the business. Mr. Stone sold low-cost, low-benefit accident insurance, underwriting and issuing policies on-site. The next year he founded his own agency, the Combined Registry Co.

As the Great Depression began, Stone reduced his workforce and improved training. Forced by his son's respiratory illness to winter in the South, Stone moved to Arkansas and Texas. In 1939 he bought American Casualty Insurance Co. of Dallas, Texas. It was consolidated with other purchases as the Combined Insurance Co. of America in 1947. The company continued through the 1950s and 1960s, continuing to sell health and accident policies. In the 1970s, Combined expanded overseas despite being hit hard by the recession.

In 1982, after 10 years of stagnation under Clement Stone Jr., the elder Stone, then 79, resumed control until the completion of a merger with Ryan Insurance Co. allowed him to transfer control to Patrick Ryan. Ryan, the son of a Ford dealer in Wisconsin, had started his company as an auto credit insurer in 1964. In 1976, the company bought the insurance brokerage units of the Esmark conglomerate. Ryan focused on insurance brokering and added more upscale insurance products. He also trimmed staff and took other cost-cutting measures, and in 1987 he changed Combined's name to Aon. In 1992, he bought Dutch insurance broker Hudig-Langeveldt. In 1995, the company sold its remaining direct life insurance holdings to General Electric to focus on consulting. The following year, it began offering hostile takeover insurance policies to small and mid-sized companies.

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 US 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.

Responding to industry demands, Aon announced its new fee disclosure policy in 1999, and the company reorganised to focus on buying personal line insurance firms and to integrate its acquisitions. That year it bought Nikols Sedgwick Group, an Italian insurance firm, and formed RiskAttack (with Zurich US), a risk analysis and financial management concern aimed at technology companies. The cost of integrating its numerous purchases, however, hammered profits in 1999.

Despite its troubles, in 2000 Aon bought Reliance Group's accident and health insurance business, as well as Actuarial Sciences Associates, a compensation and employee benefits consulting company. Later in that year, however, the company decided to cut 6% of its workforce as part of a restructuring effort. In 2003, the company saw revenues increase primarily because of rate hikes in the insurance industry. Also that year, Endurance Specialty, a Bermuda-based underwriting operation that Aon helped to establish in November 2001 along with other investors, went public. The next year Aon sold most of its holdings in Endurance.

In late 2007, Aon announced the divestiture of its underwriting business. With this move, the firm sold off its two major underwriting subsidiaries: Combined Insurance Company of America (acquired by ACE Limited for $2.4 billion) and Sterling Life Insurance Company (purchased by Munich Re Group for $352 million). The low margin and capital-intensive nature of the underwriting industry was the primary reason for the firm's decision to divest. Upon completion of the move, Aon turned its attention to expanding its broking and consulting capabilities.

This growth strategy manifested in November 2008 when Aon announced it had acquired reinsurance intermediary and capital advisor Benfield Group Limited for $1.75 billion. The acquisition amplified the firm's broking capabilities, positioning Aon one of the largest players in the reinsurance brokerage industry.

In 2010, Aon made its most significant acquisition to date with the purchase of Lincolnshire, Illinois-based Hewitt Associates for $4.9 billion. Aside from drastically boosting Aon's human resources consulting capacity and entering the firm into the business process outsourcing industry, the move added 23,000 colleagues and more than $3 billion in revenue.

In 10 February 2017, Aon announced that it is selling its employee benefits outsourcing business to Private equity firm Blackstone Group LP (BX.N) for US$4.8 billion (£3.8 billion)

Aon's New York offices were on the 92nd and 98th–105th floors of the South Tower of the World Trade Center at the time of the 11 September 2001 terrorist attack. When the North Tower was struck at 8:46 a.m., many executives began evacuating their employees from the upper floors of the South Tower. The evacuation of Aon's offices, ordered by Eric Eisenberg, was carried out quickly as 924 of the estimated 1,100 Aon employees present at the time managed to evacuate the building before United Airlines Flight 175 struck it twenty stories below them at 9:03 a.m.

However, many were influenced to stay by security guards and security announcements, or did not exit the building in time. As a result, 176 employees of Aon were killed in the attacks, including Eisenberg and Kevin Cosgrove, a vice-president of the company, who made a call to 911 when the tower collapsed at 9:59 a.m.

In 2004–2005, Aon, along with other brokers including Marsh & McLennan and Willis, fell under regulatory investigation under New York Attorney General Eliot Spitzer and other state attorneys general. At issue was the practice of insurance companies' payments to brokers (known as contingent commissions). The payments were thought to bring a conflict of interest, swaying broker decisions on behalf of carriers, rather than customers. In the spring of 2005, without acknowledging any wrongdoing, Aon agreed to a $190 million settlement, payable over 30 months.

In January 2009, Aon was fined £5.25 million in the UK by the Financial Services Authority, who stated that the fine related to the company's inadequate bribery and corruption controls, claiming that between 14 January 2005 and 30 September 2007 Aon had failed to properly assess the risks involved in its dealings with overseas firms and individuals. The Authority did not find that any money had actually made its way to illegal organisations. Aon qualified for a 30% discount on the fine as a result of its co-operation with the investigation. Aon said its conduct was not deliberate, adding it had since "significantly strengthened and enhanced its controls around the usage of third parties".

In December 2011, Aon Corporation paid a $16.26 million penalty to the US Securities and Exchange Commission (SEC) and the US Department of Justice (DOJ) for violations of the US Foreign Corrupt Practices Act (FCPA). According to the SEC, Aon's subsidiaries made improper payments of over $3.6 million to government officials and third-party facilitators in Costa Rica, Egypt, Vietnam, Indonesia, the United Arab Emirates, Myanmar and Bangladesh, between 1983 and 2007, to obtain and retain insurance contracts.

On 10 February 2017, Aon announced that it is selling its employee benefits outsourcing business to Private equity firm Blackstone Group LP (BX.N) for US$4.8 billion (£3.8 billion)

On 14 November 2016, Aon acquired CoCubes an online Indian Assessment firm, facilitating hiring of entry level engineering graduates.

On 31 October 2016, Aon's Aon Risk Solutions completed acquisition of Stroz Friedberg LLC, a specialised risk management firm focusing on cybersecurity.

On 16 June 2014, Aon announced that it agreed to buy National Flood Services, Inc., a leading processor of flood insurance, from Stoneriver Group, L.P.

On 22 October 2012, Aon announced that it agreed to buy OmniPoint, Inc, a Workday consulting firm. Financial terms were not disclosed.

On 19 July 2011, Aon announced that it bought Westfield Financial Corp., the owner of insurance-industry consulting firm Ward Financial Group, from Ohio Farmers Insurance Co. Financial terms were not disclosed.

On 7 April 2011, Aon announced that it had acquired Johannesburg, South Africa-based Glenrand MIB. Financial terms were not disclosed.

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

On 5 Mar 2010, Hewitt Associates announced that it acquired Senior Educators Ltd. The acquisition offers companies a new way to address retiree medical insurance commitments.

On 22 August 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.

On February 10, 2017, Aon PLC agreed to sell its human resources outsourcing platform for $4.3 billion to Blackstone Group L.P., creating a new company.

In September 2017, Aon announced its intent to purchase real estate investment management firm The Townsend Group from Colony NorthStar for $475 million, expanding Aon's property investment management portfolio.

On 3 June 2009, it was reported that Aon had signed a four-year shirt sponsorship deal with English football giant Manchester United. On 1 June 2010, Aon replaced American insurance company AIG as the principal sponsor of the club. The Aon logo was prominently displayed on the front of the club's shirts until the 2014/2015 season when Chevrolet replaced them. The deal was 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.

In April 2013, Aon signed a new eight-year deal with Manchester United to rename their training ground as the Aon Training Complex and sponsor the club's training kits, reportedly worth £180 million to the club.