An instrument to measure teachers' instructional practices, the Instructional Practice Questionnaire, was developed and validated in three phases. The questionnaires focused on three domains of instructional practices: cognitive, interpersonal, and interpersonal. First, an initial questionnaire was developed for a pilot study, and data were gathered to analyze questionnaire items with 97 general education teachers. Second, the questionnaire was revised based on the findings of the first-phase data analysis. Third, the revised questionnaire was distributed to two new samples (144 general education teachers and 67 GATE resource room teachers) in an effort to provide reliability and validity evidence of the scores from the revised questionnaire. The questionnaire would be useful for educators and researchers who are interested in understanding instructional practices of classroom teachers and in improving classroom instruction for gifted and talented students. Putting the research to use: Although the positive effects of differentiated education on the learning and development of gifted children have been apparent, differentiation of instructional activities has not been practiced in general education classrooms as widely as in specialized gifted programs. For teachers to provide differentiated instruction, they need to facilitate not only the cognitive development of students but also the students' social and personal growth. The questionnaire presented in this article can be used for assessing instructional practices of classroom teachers in three domains that are viewed as important for children's development-cognitive, interpersonal, and intrapersonal. As the instrument becomes available to teachers and administrators, it can serve as a useful tool for professional development, where the need for differentiated education might be underscored through discussion of items in the three domains. The questionnaire would be also useful for researchers who are interested in understanding instructional practices of classroom teachers and in improving classroom instruction for gifted children.