This article describes the design and implementation of an assessment instrument for Numbers and Operations, the first course in a program to train elementary mathematics specialists. We briefly describe the course and its content, and then we elaborate on the process we used to develop the assessment instrument and the corresponding rubric for evaluating particular items. We discuss the assessment process and its significant role in the course. We highlight particular items from the instrument to describe the gains teachers made in their understanding of fundamental mathematical ideas. We also use these items to argue for the importance of this course in helping mathematics specialists explore the content that underpins the elementary curriculum. The results are presented which indicate that course participants made significant gains as a consequence of their course experiences.