This professional development course for elementary and middle school teachers examines the three main categories in the Number and Operations strand of the NCTM's Principles and Standards of School Mathematics: understanding numbers, representations, relationships, and number systems; the meanings of operations and relationships among those operations; and reasonable estimation and fluent computation. The programs are designed both to deepen teachers' own understanding of such topics as the real number system, place value, zero and infinity, percentages, modeling operations with fractions, factors and multiples, and divisibility tests and to suggest ways to improve classroom instruction in these areas.