This document discusses collateralized debt obligations (CDOs), which are securities backed by a pool of debt obligations such as loans, bonds, and other assets. CDOs issue multiple tranches (layers) of securities with varying levels of risk and return, including senior, mezzanine, and equity tranches. CDOs provide advantages such as allowing investors to customize their credit risk exposure and take on diversified credit risk. However, CDO pricing relies on rating agencies' default probabilities, which may not accurately reflect the underlying risks. Expenses also reduce returns to investors. CDOs have become a large and fast-growing sector in asset-backed securities markets globally.