( CNN ) -- The financial world was thrown into a tailspin this week as credit ratings agency Standard & Poor 's first downgraded Greece 's debt rating to junk status , and also lowered the ratings of Portugal and Spain . Behind the headlines stand the firms that make the calls on the market , which begs a closer look at these powerful players in the global economy . Who are the credit ratings agencies ? The `` big three '' are Standard 's & Poor 's , Moody 's Investor Services and Fitch Ratings . All originated in the United States , although Fitch has dual headquarters in New York and London . What do they do ? They are designed to provide independent analysis of the credit worthiness -- the ability to pay off loans or investments -- of companies , countries and financial products . They rate them on a sliding scale ranging from AAA to D. In the case of government bonds , anything that slips to BB+ , as was the case with Greece this week , is considered a `` highly speculative '' investment : Or `` junk bonds '' in the parlance of the markets . Why do they wield such power ? Investors across the world look to credit ratings agencies to judge where to place their bets in the market . For governments , the ratings agencies have a lot of power over the popularity of bonds : cash given to governments like Greece by investors that , over time , will pay a return on the original investment -- unless the government defaults . The downgrade of Greece signaled Standard & Poor 's belief that Greece has a higher likelihood to default on investments . It caused investors to lose their appetite to invest in bonds from Greece , which imperils the nation 's ability to pay down its deficit -- which now stands more than 13 percent higher than the total economic output of the country . What Greece 's debt rating downgrade means . The decisions of credit rating agencies also have a ripple effect through the market : In the wake of the Greece downgrade , investors across the globe started rethinking investment in other governments ' bonds and began selling off more risky investments . How are they paid ? Historically , they were created to give investors an unbiased assessment of investments and investors paid for access to the ratings . In the 1970s , however , credit rating agencies started charging the issuers of new investments fees for ratings . In 1975 , U.S. legislators -- fearing a proliferation of unscrupulous ratings agencies -- designated Standard & Poor 's , Moody 's and Fitch as the only ratings organizations banks and brokers could use to evaluate the credit worthiness of their products . What are the complaints against the firms ? Critics complain the agencies have lost their ability to independently judge the risk on certain investments -- especially in light of AAA ratings given to mortgage-backed securities that imploded when defaults on U.S. home loans shot up , triggering the financial crisis . Lawmakers in the United States , the European Union and other countries around the world are now reviewing regulations on the credit ratings agencies . Credit ratings agencies , too , have revamped their procedures and have argued their ratings are merely opinions -- it 's up to the markets to decide .
Greece 's debt rating lowered to junk status , Portugal and Spain also drops . Comment: Critics say the eurozone economies are n't acting fast enough to avert crisis .