This article focuses on research findings from multiple evaluation studies on the teacher use of differentiated practices for gifted learners that have implications for best practice in gifted education internationally. Research questions centered on what specific differentiation practices were employed with gifted learners and distinctions found between levels of teaching and content areas. Using the Classroom Observation Scale-Revised (COS-R) with 329 teachers in six different school districts in four states in the Eastern United States, we report data on differentiation practices that suggest that teachers of the gifted as well as regular classroom teachers underutilized these practices, did not match them to instructional purposes, and failed to implement them as effectively as possible to have an impact on gifted student learning. Moreover, the lack of consistent use of flexible grouping practices to support instructional goals was apparent in the majority of regular classroom contexts. Findings also suggested that teachers who were using differentiated practices were using them at least somewhat effectively, suggesting that some teachers were successful in implementing best-practice strategies. The results of these multiple studies also revealed that middle school classrooms were less effective in the use of differentiation for the gifted than either elementary or high schools. A comparison of results by subject area showed that math classrooms were the most successful in implementing differentiation practices for the gifted. Overall, the study suggests that differentiated practices are underutilized in the majority of gifted program sites studied. Implications from the study suggest that more careful attention needs to be paid to the use of specific differentiation practices, found in the research literature and taught in model university gifted education training programs.