On August 5, 2011, Standard & Poor's (S&P), a prominent credit rating agency, lowered the rating of long-term U.S. government debt from AAA to AA+. The lowered rating reflects S&P's view that the risk that holders of U.S. debt will not receive interest and principal in a timely fashion has increased slightly. In the view of S&P's analysts, the recent debt ceiling agreement passed by Congress ( P.L. 112-25 , the Budget Control Act of 2011) "falls short of the amount that we believe is necessary to stabilize the general government debt burden by the middle of the decade," and (more generally) "America's governance and policymaking [has become] less stable, less effective, and less predictable than what we previously believed." In municipal and corporate debt markets, in which investors may choose among many similar bonds, a ratings downgrade usually leads to higher borrowing costs for the affected firm or governmental entity, as investors demand higher interest rates to compensate for higher perceived risk. With U.S. Treasury securities, however, the effect of a downgrade is not straightforward, because Treasuries play a unique role in the global financial system. U.S. government bonds have long been considered the "risk-free" baseline against which other investments are measured; they serve as collateral in a wide range of financial transactions; they are held by many financial institutions around the world, including central banks that use them in monetary policy and exchange rate operations; and they serve as a "safe haven" during financial crises. Even if holders of Treasury debt wished to switch to other debt instruments, no immediate substitute is available in many cases. Thus, the impact of the downgrade is hard to gauge. This report provides basic background information on Standard & Poor's and other credit rating firms and analyzes the implications of the downgrade for U.S. government finances and for the markets at large. Credit rating agencies, of which S&P, Moody's Investors Service, and Fitch Ratings are the largest, are private firms that issue ratings for thousands of bonds and other debt instruments. Ratings consist of letter grades backed by analysis of the creditworthiness of debt issuers. Although there is no legal requirement that issuers of bonds obtain a credit rating, investors place considerable reliance on the judgment of the rating agencies, and they have done so since the mid-1800s. The rating agencies were unregulated until the Credit Rating Agency Reform Act of 2006 ( P.L. 109-291 ), which was enacted partly in response to the rating agencies' failure to warn investors of the impending collapse of firms like Enron and WorldCom. The 2006 act requires certain rating agencies to register with the Securities and Exchange Commission (SEC) as "nationally recognized statistical rating organizations" (NRSROs) and to comply with various governance and disclosure requirements. Congress amended rating agency regulation with the Dodd-Frank Wall Street Reform and Consumer Protection Act ( P.L. 111-203 ), again in response to a perceived failure—this time related to bonds backed by subprime mortgages that were initially rated AAA, but suffered extremely high losses, helping to trigger the 2007-2009 financial crisis. Dodd-Frank mandated new disclosures about rating methodology and performance, increased legal liability for mistaken ratings, created a new system for assigning ratings of complex asset-backed bonds, and required federal agencies to remove references to credit ratings from their regulations. Although NRSRO operations are now extensively regulated, neither the content of ratings nor the methodology used to determine ratings is subject to government regulation. Thus, there is no governmental authority to challenge S&P's rating actions. The other major NRSROs, Moody's and Fitch, have not issued a downgrade. They have, however, issued statements of concern regarding the creditworthiness of the United States. Moody's, on August 2, 2011, confirmed the AAA rating for Treasury securities but assigned a "negative outlook." Moody's indicated the possibility of a downgrade if "(1) there is a weakening in fiscal discipline in the coming year; (2) further fiscal consolidation measures are not adopted in 2013; (3) the economic outlook deteriorates significantly; or (4) there is an appreciable rise in the US government's funding costs over and above what is currently expected." Fitch indicated in June 2011 that it would place U.S. debt on "Rating Watch Negative," indicating the possibility of a downgrade, unless the debt ceiling were raised by August 2. In the wake of the Budget Control Act, Fitch issued a statement that "despite the intensity and theatre of political discourse in the United States, there is the political will and capacity to ultimately do the right thing. In Fitch's opinion, the agreement is an important first step but not the end of the process towards putting in place a credible plan to reduce the budget deficit to a level that would secure the United States' 'AAA' status over the medium-term." According to S&P, an issuer of debt rated AAA has an "extremely strong capacity to meet its financial commitments," whereas AA issuers have "very strong" capacity. The two differ "only to a small degree." Because the United States' rating is now AA+, the degree of additional risk signaled by the rating change is very small. The rationale for the change appears to be qualitative—S&P's opinion about the prospects for political action to change the government's long-term debt path—rather than quantitative. S&P acknowledged that its initial calculations used an inappropriate Congressional Budget Office measure (an "alternate," rather than "baseline" estimate of future discretionary spending), resulting in a $2-trillion error, but declined to reconsider the rating change when Treasury analysts noticed the mistake. Thus, the downgrade does not convey any new information to the markets about the U.S. fiscal situation, beyond the pessimism of S&P's analysts themselves. In the context of unsettled global financial markets, however, the historically unprecedented downgrade may affect the confidence and perceptions of market participants and bring changes that are difficult to anticipate. Specific market sectors are linked to the Treasury bond market in different ways. One fear arising from the downgrade is that borrowing costs for the U.S. government would have to rise to compensate investors for added risk in holding Treasuries. Such an increase in interest rates could add significantly to the government's debt burden, worsening the fiscal picture. On Monday, August 8, 2011, the first trading day after the downgrade announcement, however, the yields on 10-year Treasury bonds fell; investor interest in buying them remains strong. The secondary (or resale) market for Treasury securities is very active—over half a trillion dollars per day—and each transaction represents a judgment about the government's creditworthiness. Many factors besides ratings affect bond prices, but the impact of a ratings change is generally greater when the issuer is relatively unknown to investors and where there is not an active secondary market producing a constant stream of price information. The downgrade may push yields up in the weeks to come, but S&P's action did not trigger an immediate flight out of Treasuries. Even in the long run, a lower rating may not lead to higher borrowing costs. Japan was downgraded to AA in 2002, but its government bond yields remained low, well below those of the United States. Of course, comparisons between Japan, the United States, and other nations are not simple. The S&P ratings of most corporate and municipal securities were unaffected by the downgrade. On August 8, 2011, however, S&P announced downgrades of several entities related to the U.S. government, all from AAA to AA+. Those affected were "agency" securities, obligations of the Federal Home Loan Banks, the Farm Credit System, and Fannie Mae and Freddie Mac. This action is because S&P views these entities as very likely to receive government support should they encounter financial difficulties. (In the cases of Fannie and Freddie, the promise of support was made explicit when those firms were placed under government conservatorship in 2008.) As a result, their creditworthiness is linked to that of the U.S. Treasury. The impact of the downgrade on their borrowing costs is subject to the same uncertainty as the impact on U.S. debt. In addition, certain bond issues guaranteed by the Federal Deposit Insurance Corporation and the National Credit Union Administration (as part of emergency programs related to the financial crisis) were downgraded. These downgrades simply reflect the U.S. sovereign downgrade, rather than any new information about the affected debt instruments or the issuers. Certain structured finance instruments backed by government-insured student loans or government leases were also downgraded or given negative outlooks. On August 8, 2011, S&P downgraded five large U.S. insurance groups from AAA to AA+. The firms were Knights of Columbus, New York Life, Northwestern Mutual, Teachers Insurance and Annuity Association of America (TIAA), and United Services Automobile Association (USAA). The downgrades occurred because these insurers' businesses and assets are highly concentrated in the United States and they have significant holdings in U.S. Treasury and agency securities. Five other insurance groups had their AAA ratings reaffirmed. Federally insured depository institutions hold more than $300 billion in Treasury securities. Under the Basel international capital accords, they are required to maintain capital in proportion to the riskiness of their assets. The downgrade to AA+, however, should not have an immediate impact because the Basel II standards assign a zero risk weight to sovereign debt rated AAA through AA-. In addition, many U.S. banks still follow the Basel I standards, which treat all government debt as risk-free.