Building Blocks is a National Science Foundation-funded project designed to enable all young children to build a solid foundation for mathematics. To ensure this, we used a design and development model that drew from theory and research in each phase. Our design process is based on the assumption that curriculum and software design can and should have an explicit theoretical and empirical foundation, beyond its genesis in someone's intuitive grasp of children's learning. It also should interact with the ongoing development of theory and research-reaching toward the ideal of testing a theory by testing the software and curriculum in which it is embedded. Our model includes specification of mathematical ideas (computer objects or manipulatives) and processes/skills (software ''tools'' or actions) and extensive field-testing from the first inception through to large summative evaluation studies. The initial field test results indicate that such an approach can result in significant assessed learning gains consistent with the new Principles and Standards for School Mathematics of the National Council of Teachers of Mathematics.