The objective of this chapter is to describe the evaluation methods, techniques, and tools involved in measuring implementation fidelity. Implementation fidelity is defined and the distinction between process evaluation and formative evaluation is described. The body of knowledge known as implementation research is introduced, and the National Implementation Research Network’s Implementation Drivers Framework and implementation stages are presented. The use of manuals, protocols, checklists, and practice profiles to support high-fidelity practitioner behaviors are illustrated. Research-validated tools for assessing implementation fidelity within a multi-tiered system of support initiative are highlighted. This chapter concludes with a discussion regarding the use of established criterion for judging implementation fidelity and the critical importance of measuring implementation fidelity linked to intended student outcomes.