This project seeks to enable teachers to fully-leverage interactive simulations to advance student engagement and learning of mathematics, moving technology from the margins to a core part of instruction. The project studies: How the design of an interactive simulation can generate pedagogically-productive use? How instruction with simulations can be best structured to support learning of mathematical concepts and engagement in mathematical practices? How sim-based instruction can be made attractive, feasible and effective for teachers? How student learning is impacted by sim-based instruction?