People believe mathematics is important. It is taken for granted that numeracy is one of the basics of a sound education. Poor math skills are a worry all over the world, and there is growing consensus around a few key beliefs: math "skills" involve more than memorization of number facts and formulae. Teaching for understanding is important. Being good at math means being able to solve problems. Math educators once again tackled the problem of teaching abstract mathematical ideas in ways that would develop sound understanding: One focus of the new math was set theory, where students were encouraged to think of numbers in a new, hopefully more concrete way. Meaningful mathematical reform must start with the classroom. If change is to take hold, teachers and students alike have to accept, understand, and enact it within the context of what they already know about mathematics.