A principal may have implemented a program that boasted glowing reports but didn't live up to its promise. The cause of this disparity could be that the program didn't match the school's and students' needs in terms of the curricular focus or that previous results were more positive than warranted. However, another possibility could be that the programs that research demonstrates can have positive impacts on student learning have not been put into practice in the way developers had intended. A program or approach that is effective in other settings can be ineffective in a school if the way it is implemented takes it far away from its original design. The term used to describe this concept is fidelity of implementation (FOI)--"the delivery of instruction in the way in which it was designed to be delivered." In this article, the author discusses why FOI is important, identifies the four factors that affect FOI (Complexity; Required Materials and Resources; Credibility; Interventionists), and describes strategies for making FOI work. (Contains 3 online resources.)