This article reports on the evaluation of a model for assessment of content knowledge used by researchers in the Problem-Based Learning (PBL) Project for Teachers of Science. These assessment strategy and scoring methodology result in scores for each teacher about the quality of their understanding of each Big Idea before and after PD. The compilation of scores by teacher facilitates assessment of the strength of teachers’ incoming knowledge and changes in their knowledge both in terms of number of Big Ideas and the clarity, accuracy, and completeness of that use. The assessment strategy offers advantages and limitations of this method.