Aon is a professional services firm that makes money by acting as a middleman between businesses and the world of insurance and risk. When a large company needs to insure a skyscraper, protect against a cyberattack, or figure out how to structure its employee benefits, it often hires Aon. Aon earns a commission on the insurance premium placed, or charges a fee for its advice. It does this across two main businesses: Risk Capital, which covers insurance and reinsurance brokerage, and Human Capital, which covers health benefits, retirement consulting, and workforce advice. In 2025, Risk Capital brought in $11.3 billion and Human Capital brought in $5.9 billion of Aon's total $17.2 billion in revenue. No single client accounts for more than about 1% of that total, which means the business is spread across thousands of relationships worldwide. The diagram below traces where the money goes.
Five years of financial data tell a clear story of growth, but with a notable complication at the bottom of the balance sheet. Revenue rose from $12.2 billion in 2021 to $17.2 billion in 2025. That is a gain of $5 billion over four years. Organic revenue growth, which strips out the effect of acquisitions and currency moves, ran at 6% in both 2024 and 2025. That means the existing business is genuinely winning new clients and holding onto old ones, not just growing by writing checks for other companies.
Free cash flow, which is the actual cash left over after running the business and paying for equipment, also grew across the period. It was $2.0 billion in 2021 and reached $3.2 billion in 2025. That is the kind of real cash generation that lets a company pay dividends, reduce debt, or fund future deals. But net debt tells a more complicated story. It stood at $8.8 billion in 2021, climbed steadily, and then jumped sharply to $15.9 billion in 2024 after Aon spent heavily to acquire NFP. By 2025 it had pulled back to $14.1 billion, suggesting Aon is beginning to pay that debt down. The operating margin also moved in the right direction, rising from 24.4% in 2024 to 25.3% in 2025, with restructuring savings from the Accelerating Aon United Program contributing $160 million of that improvement.
The risks Aon faces are specific and worth naming clearly. The first is structural. A growing share of Aon's commissions depends on the size of insurance premiums, which Aon does not set. If large clients decide to self-insure, use their own captive insurance vehicles, or find capital market alternatives to traditional insurance, Aon earns less even if its advice is exactly as valuable. The second risk is competitive. Aon has invested heavily in data and analytics tools, including artificial intelligence capabilities. But rivals like Marsh McLennan and Willis Towers Watson are doing the same. If Aon's tools do not stay ahead, clients may see less reason to pay a premium for Aon's advice over a cheaper alternative.
A third risk is tax. Aon operates across more than 120 countries and has historically structured itself to manage its global tax rate. The new OECD Pillar Two minimum tax, already enacted in Ireland, the UK, Singapore, and across much of Europe, introduces real uncertainty about what Aon's future tax bills will look like. Aon's own filings describe the situation as having significant uncertainty about how the rules apply to prior years and future years. A higher effective tax rate would directly reduce the cash that flows through to shareholders. The fourth risk is currency. More than half of Aon's revenue, specifically 51.8%, comes from outside the United States. A stronger US dollar quietly shrinks those numbers when converted back to dollars, even if the underlying business in each country is performing well.
There is also a legal tail risk worth noting. Aon gives advice on complex insurance placements and consulting matters. When that advice turns out to be wrong, or a client believes it was wrong, Aon faces errors and omissions claims. The company's own filings state that some of its insurance coverage for such claims has already been exhausted, meaning Aon is now effectively self-insuring against historical claims. Future claims of meaningful size could come directly out of earnings.
The NFP acquisition is the central fact of the current investment case. Aon paid a price that nearly doubled its debt load. It then sold the NFP Wealth business in late 2025, banking a large gain and reducing some of the exposure. What remains is whether the rest of NFP, absorbed into Aon's Risk Capital and Human Capital segments, actually earns back the cost of that debt over time. Human Capital's adjusted operating margin moved from 29.5% in 2024 to 32.2% in 2025, which suggests integration is progressing. But $14.1 billion in net debt against $3.2 billion of annual free cash flow leaves little room for error.