This Practice Note explains collateralised debt obligations (CDOs) and the applicable UK regulatory framework. It covers (1) core concepts relevant to CDOs including special purpose vehicles (SPVs), securitisation, tranches and the creation of security over a portfolio of financial assets which may include asset-backed securities (ABS), mortgage-backed securities (MBS) and other issues of CDO securities, (2) the principal parties involved in a CDO transaction (arranger, portfolio manager, rating agencies, issuer and investors), (3) the main types of CDO structures (cash flow CDO, market value CDO and synthetic CDO), (4) the main types of portfolio management structures (dynamic and static), (5) the capital structure of SPVs used for CDO transactions, (6) the use of hedging in CDO structures and (7) key considerations and legal issues relevant to CDOs (bankruptcy remoteness, methods of transferring the underlying assets to the SPV, jurisdiction and tax issues, credit enhancement and overcollateralisation). What is a CDO? Core concepts Collateralised debt obligations (CDOs) are complex, high-value transactions involving numerous parties, extensive documentation and, usually, several jurisdictions. A CDO transaction involves an orphan shell company (known