Credit rating and analysis of collateralized debt obligations and all structured finance products are integral to the smooth function of the secondary market for mortgage loans. A credit rating agency is a company that analyzes issuers of debt and debt-like securities and gives them an overall credit rating which measures the issuer's ability to satisfy its debt obligations.

There are more than 100 major rating agencies around the world, and three of the largest and most important ones in the United States are Fitch Ratings, Moody's and Standard