For 25 years, we have been working to build cognitive models of mathematics, which have become a basis for
middle- and high-school curricula. We discuss the theoretical background of this approach and evidence that
the resulting curricula are more effective than other approaches to instruction. We also discuss how embedding
a well specified theory in our instructional software allows us to dynamically evaluate the effectiveness of our
instruction at a more detailed level than was previously possible. The current widespread use of the software is
allowing us to test hypotheses across large numbers of students. We believe that this will lead to new approaches
both to understanding mathematical cognition and to improving instruction.