A collateralized debt obligation (CDO) is a type of structured asset-backed security (ABS). Originally developed as instruments for the corporate debt markets, after 2002 CDOs became vehicles for refinancing mortgage-backed securities (MBS). Like other private label securities backed by assets, a CDO can be thought of as a promise to pay investors in a prescribed sequence, based on the cash flow the CDO collects from the pool of bonds or other assets it owns. Distinctively, CDO credit risk is typically assessed based on a probability of default (PD) derived from ratings on those bonds or assets.