In this paper, the author describes the case of a teacher, Mark Black, as he struggles to adapt to the calls for the reform of teaching in California. Drawing on a set of interviews and observations that are part of a larger study (see Cohen, Ball, and Peterson in this volume), the author explores how Mark enacts the curriculum of Real Math, the textbook that his school district recently adopted. Through the lenses of his beliefs about the nature and structure of mathematical knowledge, his beliefs about how students best learn mathematics, and his beliefs about his role as a teacher, Mark transforms the innovative textbook into a more familiar, traditional elementary mathematics curriculum. The author discusses four real and perceived constraints that influence Mark's ability to enact the curricular policy proposed by the Framework and argues that teachers are themselves learners who need to be supported and nurtured as they try to change their practice.