Misconceptions and incomplete mathematical thinking often go undetected, as teachers are rarely trained to consider why or how an answer was generated. They often respond to incorrect answers by re-teaching the correct processes, and the underlying problem persists. Unrecognized misconceptions then become permanent ways of thinking; each new layer of mathematics knowledge builds upon on flawed foundations. This book addresses that problem head on. By covering the essential content or "big ideas" of K-2 mathematics, it turns elementary teachers into experts in diagnosis, intervention, and evaluation, and delivers the tasks and tools to use in the classroom day-to-day.