His research interests focus on improving teaching and learning in mathematics classrooms. He has investigated students’ learning in a variety of instructional environments, including a four-year longitudinal study comparing primary grade students receiving conceptually-based instruction with students receiving procedurally-oriented instruction. As director of the 1999 TIMSS-R mathematics video study, he was involved in a large-scale study comparing eighth-grade mathematics teaching in six high-achieving countries with teaching in the United States.

Most recently, he has focused the effects of improving the mathematics preparation of K-6 teachers. Two longitudinal studies have followed graduates of the University of Delaware program into their first years of teaching. Underlying much of this work is an interest in the process of continuous improvement of mathematics teaching and its long-term effects on the learning of students and teachers. Reports of this work can be found in the series of publications linked to this website.