Rating agencies are companies that assign credit ratings to issuers of securities, such as bonds or stocks. These agencies use complex mathematical models and analytical methods to assess an issuer's ability to repay its debt obligations in a timely manner. The resulting rating is used by investors to evaluate the risk associated with a particular security and make informed decisions about whether to purchase it. Some of the largest and most well-known rating agencies include Moody's, Standard & Poor's (S&P), and Fitch Ratings.