Educational reformers increasingly seek to manipulate policies regarding assessment, curriculum, and professional development in order to improve instruction. They assume that manipulating these elements of instructional policy will change teachers' practice, which will then improve student performance. We formalize these ideas into a rudimentary model of the relations among instructional policy, teaching, and learning. We propose that successful instructional policies are themselves instructional in nature: because teachers figure as a key connection between policy and practice, their opportunities to learn about and from policy are a crucial influence both on their practice, and, at least indirectly, on student achievement. Using data from a 1994 survey of California elementary school teachers and 1994 student California Learning Assessment System (CLAS) scores, we examine the influence of assessment, curriculum, and professional development on teacher practice and student achievement. Our results bear out the usefulness of the model: under circumstances that we identify, policy can affect practice, and both can affect student performance.