The U.S. school mathematics curriculum changed slowly during the last century, and only in the past few decades have there been serious efforts to establish it nationally rather than having it be, at least in principle, locally determined. Waves of change have periodically swept the curricular ocean, but on the seabed, in the classrooms where students encounter their curriculum, much less has changed. A variety of recent efforts have been undertaken to change the curriculum nationally, and the country may be close to adopting a common core of standards for the school mathematics curriculum. In view of the complexity of how the curriculum operates and how it might change--and in particular, in view of the critical role teachers of mathematics play in making any change--the effects a national curriculum might have on classroom practice are impossible to predict.