In this chapter, a model for valuing the loan book of a mortgage credit institute (MCI) is presented. An MCI’s loan book generates a cash flow that stems from borrowers paying an administration fee on top of the coupon on their mortgages. The MCI bears all credit risk associated with its business, and the administration fee must therefore, in addition to covering the costs of administering the loans and return demands, also cover credit risk. Therefore, this cash flow must be assessed while considering the risk that borrowers will not be able to meet their obligations. For that reason, techniques from the credit derivatives area are used. In this simple model, the fair value of a single loan in the loan book is given by summing payments on the basis of the differences between the paid administration fee and a fair administration fee, where the fair administration fee is the fee that gives a loan fair value of zero. A fair value of zero means that all costs, including demand for return and credit risk premiums, are exactly covered by the administration fee. Various results are presented, especially effects on the loan book value as a result of changing property values.