Students ask a variety of questions during mathematics lessons. Some questions focus on procedures or formal notation, while others focus on ideas, connections, or representations. Student questions are important as they provide vastly different learning opportunities for students. In order to support teachers as they help students learn to ask productive questions in mathematics lessons, we need to first understand the types of questions students ask. This paper describes the process of creating a categorization for student questions in middle and high school mathematics classrooms. [For the complete proceedings, see ED583608.]