Credit rating agencies evaluate how likely a borrower is to repay its debt and publish that judgment as a letter grade, from AAA at the top down to D for default. What credit rating agencies do, in practical terms, is analyze the finances of governments, corporations, and pooled securities and translate that analysis into a shorthand the whole market can use to price risk. Three firms dominate the field: S&P Global Ratings, Moody’s Investors Service, and Fitch Ratings. Eleven firms are currently registered with the SEC as Nationally Recognized Statistical Rating Organizations, but the Big Three issue most of the ratings that move global markets.1U.S. Securities and Exchange Commission. Current NRSROs They do not rate individual consumers. Your personal credit score comes from a separate system.
What They Rate
Rating agencies cover the large-scale institutional borrowing that fills bond markets. The work falls into three broad categories.
Sovereign debt. When a national government issues bonds, agencies assess its ability and willingness to repay. The analysis weighs macroeconomic performance, political stability, institutional strength, and whether the tax base can support the debt load. A sovereign rating tends to function as a practical ceiling for other borrowers in the same country, since a shaky government usually drags domestic banks and corporations into the same risk conversation.
Corporate and municipal bonds. For a company, the analyst studies financial statements, cash flow, competitive position, and industry outlook. Municipal bonds, issued by states and localities to finance roads, schools, and other infrastructure, get similar treatment with a focus on the local tax base, revenue stability, and the legal protections written into the bond.
Structured finance. Asset-backed securities, mortgage-backed securities, and collateralized loan obligations pool hundreds or thousands of underlying loans and slice the pool into layers, called tranches, with different payment priorities. The agency assesses the collateral, the legal structure isolating the assets, and how cash flows through each tranche under stress. Because these products are engineered rather than organic, the rating depends heavily on the modeling assumptions, which is a fact worth remembering.
How a Rating Is Produced
The process starts when an issuer asks for a rating and agrees to share its financial records. Under the dominant issuer-pays model, the entity being rated pays for the analysis, and the finished rating is then published free to the market.
A lead analyst pulls together the quantitative work: balance sheets, debt ratios, cash flow projections, and any confidential information the issuer provides. That gets paired with a qualitative read on management, competition, industry trends, and the wider economy. For structured products, the analyst also models how the security would hold up under scenarios like rising defaults or falling recoveries.
The recommendation then goes before an internal rating committee, a group of experienced professionals who debate the evidence, challenge assumptions, and vote. Before publication, the issuer can flag factual errors but cannot lobby for a better grade. Once the rating is out, it isn’t frozen. Analysts monitor the issuer’s condition, the industry, and the regulatory environment, and can place the rating on watch and eventually upgrade or downgrade it.
Reading the Letter Grades
Each agency uses a slightly different scale, but the message is the same: how likely the borrower is to default.
S&P and Fitch
Both scales start at AAA and step down through AA, A, BBB, BB, B, CCC, CC, C, and D for default.2S&P Global. Understanding Credit Ratings A plus or minus sign adds granularity within each letter: AA+ sits just below AAA, and AA- at the bottom of the AA range.3Fitch Ratings. Rating Definitions
Moody’s
Moody’s top rating is Aaa, followed by Aa, A, Baa, Ba, B, Caa, Ca, and C. Instead of plus and minus signs, Moody’s uses numerical modifiers: 1 for the higher end of a category, 2 for the middle, 3 for the lower. Aa1 at Moody’s is roughly equivalent to AA+ at S&P or Fitch.4Moody’s Investors Service. Moody’s Rating Symbols and Definitions
Investment Grade Versus Speculative Grade
The most consequential line on the scale separates investment grade from speculative grade. At S&P and Fitch, BBB- and above is investment grade; BB+ and below is speculative.2S&P Global. Understanding Credit Ratings At Moody’s, the cutoff falls between Baa3 (investment grade) and Ba1 (speculative).4Moody’s Investors Service. Moody’s Rating Symbols and Definitions Speculative-grade debt is often called high-yield or junk. The higher yields exist because investors demand extra return for the greater chance they won’t get paid back.
Outlooks and Watch Lists
Alongside the letter grade, agencies assign an outlook (positive, negative, stable, or developing) to signal where they think the rating is headed over the next one to two years. When something more immediate is unfolding, such as a pending merger or a sudden liquidity crunch, the agency may place the rating on credit watch, indicating a decision within weeks rather than months.
Why Ratings Matter
A rating is a letter, but it functions as a key that opens or closes access to enormous pools of capital.
Borrowing Costs
The link between a rating and borrowing costs is direct. A company rated AA can issue bonds at a lower interest rate than one rated BBB, because investors see less risk and accept less return. For a large issuer floating billions in debt, a single one-notch downgrade can mean tens of millions of dollars in additional annual interest expense. Sovereign downgrades work the same way, raising the cost of funding everything from infrastructure to social programs.
Regulatory Gatekeeping
Many institutional investors face legal or regulatory limits on the kinds of bonds they can hold. Banks, for example, must demonstrate that their securities holdings meet an investment-grade standard, meaning the bank has determined the issuer’s capacity to meet its commitments is adequate and default risk is low.5Board of Governors of the Federal Reserve System. SR 12-15 – Investing in Securities Without Reliance on Nationally Recognized Statistical Rating Organization Ratings Pension funds and insurance companies operate under similar constraints. Federal regulators have moved away from blind reliance on agency ratings, but in practice, those ratings still weigh heavily in the internal creditworthiness judgments that determine what a regulated investor can own.
Fallen Angels
When a bond drops from the lowest rung of investment grade (BBB- or Baa3) to the top of speculative grade (BB+ or Ba1), the industry calls it a fallen angel. Institutional investors whose mandates prohibit speculative-grade debt are forced to sell, flooding the market with supply exactly when demand is drying up. The price drop can be dramatic, though research from the European Central Bank found that markets often begin repricing the risk before the official downgrade lands.6European Central Bank. Understanding What Happens When Angels Fall
Market Liquidity
Beyond any single upgrade or downgrade, the mere existence of a rating makes a bond easier to trade. A standardized risk assessment gives buyers and sellers a common starting point for pricing. Unrated bonds tend to trade less often and at wider spreads because the information gap makes investors cautious.
The Conflict at the Heart of the Business
The core tension in the ratings industry is that the entity being graded is also the one writing the check. Issuer-pays makes ratings freely available to the market, which is a genuine public good, but it also creates an incentive problem that has never been fully solved.
The concern is straightforward. If an issuer is unhappy with a preliminary rating, it can approach a competitor, which pressures agencies to be generous rather than lose the fee. In structured finance, where a small number of large investment banks generate enormous deal volumes, that pressure is especially concentrated. Agencies rating structured products have also sometimes advised on how to structure deals to reach higher ratings, blurring the line between evaluator and collaborator.
The 2008 financial crisis made these conflicts impossible to ignore. In the years before the crash, rating agencies gave top-tier grades to complex mortgage-backed securities and collateralized debt obligations built on increasingly risky subprime loans. When housing collapsed, those ratings proved wildly optimistic. Roughly 90 percent of the residential mortgage-backed securities issued in 2006 and 2007 were eventually downgraded from investment grade to speculative grade. Securities that had carried AAA ratings, supposedly the safest debt in the market, suffered losses that no AAA rating should have contemplated. In 2015, S&P agreed to pay $1.375 billion to settle federal and state claims that it had inflated ratings to win business from the banks packaging those securities.
Ratings carry no legal warranty of accuracy. Agencies have historically argued, and courts have sometimes agreed, that their ratings are opinions protected under the First Amendment. That defense has limits. At least one federal court has held that ratings distributed privately to a narrow group of investors rather than the public at large amount to commercial speech entitled to reduced constitutional protection.
Who Oversees the Agencies
For decades, rating agencies operated with little formal oversight. The Credit Rating Agency Reform Act of 2006 established the NRSRO registration framework and gave the SEC authority to examine agencies and enforce compliance. Agencies must disclose their methodologies, maintain policies to prevent misuse of nonpublic information, and report conflicts of interest.7Office of the Law Revision Counsel. 15 U.S. Code 78o-7 – Registration of Nationally Recognized Statistical Rating Organizations
The Dodd-Frank Act of 2010 expanded these requirements after the crisis. Section 932 mandated that at least half of each agency’s board be independent of the ratings business, with a portion representing investors who rely on ratings. Agencies must also disclose the historical performance of their ratings, the assumptions most likely to affect a rating if wrong, and the expected probability of default for each rated security.8U.S. Congress. Public Law 111-203 – Dodd-Frank Wall Street Reform and Consumer Protection Act Dodd-Frank also directed federal regulators to scrub their own rules of any blanket reliance on ratings, so banks and other regulated investors have to make independent credit judgments rather than outsource the call to a private firm.5Board of Governors of the Federal Reserve System. SR 12-15 – Investing in Securities Without Reliance on Nationally Recognized Statistical Rating Organization Ratings
For structured products, SEC Rule 17g-5 requires that a hired rating agency maintain a password-protected website listing every deal it is in the process of rating, and give other registered agencies access to the same deal information so they can produce competing ratings.9eCFR. 17 CFR 240.17g-5 – Conflicts of Interest The SEC’s Office of Credit Ratings conducts at least one examination of each NRSRO every year.7Office of the Law Revision Counsel. 15 U.S. Code 78o-7 – Registration of Nationally Recognized Statistical Rating Organizations The SEC’s January 2025 staff report documented material deficiencies at several agencies, including analysts voting on ratings while holding the issuer’s securities and sales-side employees pressuring analysts on rating actions.10U.S. Securities and Exchange Commission. 2024 Staff Report on Nationally Recognized Statistical Rating Organizations Oversight is ongoing and imperfect, and the conflicts the 2006 and 2010 reforms tried to address are still being worked through.
What a Rating Is Not
Every rating agency emphasizes the same disclaimer: a credit rating is not a recommendation to buy, sell, or hold any security. It is an opinion about default probability, not a judgment about whether a bond is a good investment at its current price. A AAA-rated bond trading at a premium can be a worse deal than a BB-rated bond trading at a deep discount. The rating tells you about the odds of getting paid back, not about relative value.
Ratings also move on a different clock than markets do. An agency might take months to downgrade a deteriorating issuer while the bond market reprices the risk in days. That lag is by design, since agencies aim for rating stability rather than real-time tracking, but it means the market often prices in bad news long before the letter grade catches up. Reading a bond’s yield spread alongside its rating gives you a fuller picture than the rating alone.