Mortgage-backed securities (MBSs) have played an important role in the housing and financial markets, providing liquidity to mortgage originators, offering investment opportunities for investors, and helping to set minimum mortgage underwriting standards. This chapter provides an overview of MBSs as an investment tool by presenting an analysis of the MBS market, discussing the securitization process, describing the main MBS pool characteristics, and examining the different types of MBSs in terms of underlying loans (residential mortgage-backed securities and commercial mortgage-backed securities), maturity, interest rate terms, pass-through of interest and principal (pass-through securities versus collateralized mortgage obligations) and issuers (private-label versus agency MBS). The chapter also highlights the major risks inherent to MBSs, particularly prepayment and credit risks.