The mathematics teacher is responsible for creating an intellectual environment in which serious engagement in mathematical thinking is the norm, for the environment of the classroom is foundational to what students learn. More than just a physical setting with desks, bulletin boards, and posters, the classroom environment forms a hidden curriculum with messages about what counts in learning and doing mathematics: Neatness? Speed? Accuracy? Listening well? Being able to justify a solution? Working independently? If we want students to learn to make conjectures, experiment with alternative approaches to solving problems, and construct and respond to others' mathematical arguments, then creating an environment that fosters these kinds of activities is essential.