SimCalc is an educational software and curriculum program designed to introduce students as young as middle school age to fundamental mathematical concepts—change and variation—that underpin the transition from algebra to calculus. The core underlying mathematical idea is the Fundamental Theorem of Calculus, and through activities involving change and variation, SimCalc students acquire contextualized, networked, and collaborative experience with the relationship between derivatives and antiderivatives. The program had been guided from its birth by the late James J. Kaput, a mathematics education leader who thrived by working on the leading edge of the field. This book reports not only on the theory on which SimCalc is based but also on more than 15 years of small-scale and large-scale research on the impact of SimCalc. It also includes thoughtprovoking discussions of the ways in which the SimCalc approach relates to other work on engaging students in mathematical thinking.