In this chapter we explore the traditional and prevalent approach to credit risk assessment—the credit rating system. Most rating systems are based on both quantitative and qualitative evaluations. The final decision is based on many different attributes, but usually it is not calculated using a formal model that would show how to weight all these attributes in a normative way. In essence, the systems are based on general considerations and on experience, and not on mathematical modeling. They cannot therefore be regarded as precise, and they also clearly rely on the judgement of the ratings evaluators.