Fidelity-of-implementation (FOI) measures are a necessary part of rigorous efficacy and effectiveness studies. This chapter discusses tools for measuring implementation that provide data that specifically describe how teachers are using programs. It demystifies the notion of FOI by demonstrating its use for practitioners who have both large- and small-scale questions to answer. FOI begins with articulating the program model so that users know what is or is not being enacted and in what ways it is being enacted. The chapter further talks about the FOI framework, and FOI instruments and use scenarios. The level-one schools were the sites of the most intensive data-collection efforts. The level-two scenario focused on using the instruments that required teachers to spend minimal time outside the regular working day to participate in data collection. Five use cases presented here illustrate the FOI and provide potential ideas on the process of using the FOI instruments.