One of the characteristics of effective science teachers is a deep understanding of science concepts. The ability to identify, explain and apply concepts is critical in designing, delivering and assessing instruction. Because some teachers have not completed extensive courses in some areas of science, especially in middle and elementary grades, many professional development programs attempt to strengthen teachers' content knowledge. Assessing this content knowledge is challenging. Concept inventories are reliable and efficient, but do not reveal depth of knowledge. Interviews and observations are time-consuming. The Problem Based Learning Project for Teachers implemented a strategy that includes pre-post instruments in eight content strands that permits blind coding of responses and comparison across teachers and groups of teachers. The instruments include two types of open-ended questions that assess both general knowledge and the ability to apply Big Ideas related to specific science topics. The coding scheme is useful in revealing patterns in prior knowledge and learning, and identifying ideas that are challenging or not addressed by learning activities. The strengths and limitations of the scoring scheme are identified through comparison of the findings to case studies of four participating teachers from middle and elementary schools. The cases include examples of coded pre- and post-test responses to illustrate some of the themes seen in teacher learning. The findings raise questions for future investigation that can be conducted using analyses of the coded responses.