In this article, we argue that the debates about mathematics education that have arisen in the United States over the past decade are the result of a major shift in how we conceptualize mathematical knowledge and mathematics learning. First, we examine past efforts to change mathematics education and argue they are grounded in a common traditional paradigm. Next, we describe the emergence of a new paradigm that has grown out of a coalescence of theories from cognitive psychology, an awareness of the importance of culture to learning, and the belief that all students can and should learn meaningful mathematics. Reforms grounded in the new paradigm have the potential to dramatically alter the way in which students--as well as which students--experience success in school mathematics. We discuss some implications of these reforms related to how mathematics educators might work with teachers of mathematics.