The concept of "implementation fidelity" is broadly used to capture the extent to which an intervention is executed as intended by the designers of the intervention (Century, Rudnick, & Freeman, 2010; Huntley, 2005, McNaught, Tarr, & Sears, 2010, Munter, 2010). Though implementation fidelity instruments are often used to assess variability in teachers' implementations of an intervention and can be related to measures of student learning, the form and goals of the work involved in developing, implementing, and evaluating fidelity instruments vary from study to study. This research report discusses two research projects that shed light on two challenges associated with assessing implementation fidelity.