Many models are available for modeling the short rate—that is, the forward rate for the short time period—and many papers and books have been written on these models. These frameworks, however, often involve intimidating mathematics, and the lack of guidance in the extant literature makes implementing them even more challenging. To address such problems, this monograph explains many of the financial and mathematical features of no–arbitrage models. The authors develop the models and the mathematical equations and approximations for them, present the strengths and weaknesses of the models, and indicate how they can be used to model the short rate.