Publisher Summary This chapter discusses LM317, which is the first voltage regulator integrated-circuit (IC) to implement the concept of using the ratio of two resistors to define a desired output. It presents the first approximation equation to calculate the output voltage for LM317. The instructions for installing the Nearest Values add-in are provided. The functions in this add-in allow looking up standard resistor values. Various steps to develop the Excel model for LM317 are described. Some functionality is added to prompt the user for the desired output voltage by using InputBox function. The chapter also presents worst case analysis that finds the possible variation range of the output voltage based on the tolerances of the resistors and the LM317. Development of the model through stages has lead to some inconsistencies in data entry and data flow.