William Jacob is a professor of mathematics at the University of California, Santa Barbara. In addition to his mathematical research he develops and teaches courses for undergraduates preparing for teaching careers. Over the past fifteen years he has designed and led numerous professional development programs for K-12 teachers. He has been a collaborator with Mathematics in the City for five years and was a field test partner for the development of the CD-ROMs. He recently coauthored, with Catherine Fosnot, Constructing Algebra, the fourth book in the Young Mathematicians at Work series from H...