The purpose of this study is two-fold: first, to explain the demise of subprime and Alt-A mortgage markets in the U.S. from the viewpoint of measuring and managing mortgage credit risk; and secondly, to discuss several policy lessons that can be learned from the market meltdown. To that end, three tiers of mortgage credit models are elaborated, including the scoring (or risk rank-ordering), risk-based pricing, and ��sizing�� (or the analytics used in determining subordination levels of credit-sensitive mortgage backed security (MBS) deals) models. Using these as conceptual underpinning, empirical evidence is surveyed to document key contributing factors to the market demise. Those that are identified include the non-availability of reliable mortgage performance data, lack of theory as well as industry best-practices in performing simulation-based mortgage risk assessments, complex and arcane structures of mortgage backed securities, and information asymmetry among the parties involved in the security transactions. The overall conclusion derived is that the participants to these market segments surpass their risk management capabilities in globalizing funding for subprime and Alt-A mortgages. The policy lessons emphasized are the importance of the infrastructure of proper risk assessment and risk-based pricing, as well as prudent and transparent MBS products along with periodic information disclosure.