A Challenge of Numbers describes the circumstances and issues centered on people in the mathematical sciences, principally students and teachers at U.S. colleges and universities. A healthy flow of mathematical talent is crucial not only to the future of U.S. mathematics but also as a keystone supporting a technological workforce. Trends in the mathematical sciences' most valuable resource—its people—are presented narratively, graphically, and numerically as an information base for policymakers and for those interested in the people in this not very visible, but critical profession.