Commercial mortgage-backed securities (CMBS) are a type of mortgage-backed security backed by commercial and multifamily mortgages rather than residential real estate. CMBS tend to be more complex and volatile than residential mortgage-backed securities due to the unique nature of the underlying property assets.

CMBS issues are usually structured as multiple tranches, similar to collateralized mortgage obligations (CMO), rather than typical residential "passthroughs." The typical structure for the securitization of commercial real estate loans is a real estate mortgage investment conduit (REMIC), a creation of the tax law that allows the trust to be a pass-through entity which is not subject to tax at the trust level.

Many American CMBS transactions carry less prepayment risk than other MBS types, thanks to the structure of commercial mortgages. Commercial mortgages often contain lockout provisions (typically a period of 1-5 years where there can be no prepayment of the loan) which they can be subject to defeasance, yield maintenance and prepayment penalties to protect bondholders. European CMBS issues typically have less prepayment protection. Interest on the bonds may be a fixed rate or a floating rate, i.e. based on a benchmark (like LIBOR/EURIBOR) plus a spread.