Over 5 million students are learning English in America's public schools, accounting for more than 10% of the K-12 population. That's a 50% increase in the last decade. This demographic change has been matched by changes in national policy. Before No Child Left Behind, states set their own accountability policies. Now, they must demonstrate that English learners are making progress in English and achieving challenging academic content standards. Assuring that English learners succeed has proven to be much more challenging than most educators assumed. The challenge appears to be rooted in what it means to learn English in school. (Contains 2 figures.)