Bank regulation has made increasing use of external credit ratings in recent years and, in June 1999, the Basel Committee on Banking Supervision proposed a revision to the international capital accord that included a more prominent role for credit ratings. This study contains background information about credit ratings and complementary sources of credit quality information, which may be helpful to those revising the proposed accord and more generally to those interested in the use of credit ratings in regulation. The focus of the study is on factual information, rather than on subjective assessments or explicit policy recommendations. The material may be classified into four basic categories: