This study reports on the development and field study of K-TEEM, a web-based assessment instrument designed to measure mathematical knowledge for teaching (MKT) at the early elementary level. The development process involved alignment with early elementary curriculum standards, expert review of items and scoring criteria, cognitive interviews with practicing teachers, a field test involving 405 practicing teachers, and data modeling using a Rasch model. Several examples of MKT at the early elementary level are provided, and some of the challenges and decisions made during the process of item and scale development are discussed. Rasch model results indicate good model fit and adequate reliability, and the model accounts for more than 75% of the variance in the data. The K-TEEM assessment instrument may fill an important gap in the set of tools available to researchers for program evaluation and empirical investigation of teacher knowledge. [For the full text grantee submission, see ED575171.]