Scholars have long argued that individuals' beliefs influence their behaviors and the decisions they make throughout their lives. Focusing on beliefs as a cognitive construct, the purpose of this study was to identify several key beliefs about mathematics teaching and learning held by practicing elementary mathematics teachers. An iterative process of literature review, item development and adaptation, expert review of items, and cognitive interviews resulted in 55 items and 5 hypothesized belief constructs. After using the items in a questionnaire completed by more than 200 practicing teachers in two waves of data collection, we modeled the response data using a multiphase process in pursuit of parsimony and a clear factor structure. The resulting 21-item questionnaire provides an alternative measure of Transmissionist beliefs about teaching and a first way to measure two new constructs in teacher beliefs research: Facts First and Fixed Instructional Plan. [For the corresponding grantee submission, see ED599648.]