This article explores the potential of using students' science notebooks to help elementary teachers monitor conceptual understanding. Data came from 25 teachers and their 4th- and 5th-grade students working on a unit on electricity. At the time data were collected, 8 of the teachers had received professional development on science content knowledge, unit learning goals, assessing student work, and feedback. Measures included conceptual understanding of circuits from students' notebooks, a performance task, and a multiple-choice test. Classes of teachers with and without professional development are compared on these measures, and notebooks of students in trained teachers' classes are examined in some detail. Qualitative data from teacher interviews and observations of professional development sessions help explain the findings and identify teacher support needs. Notebooks have potential for formative assessment, but this use is limited by the guidance teachers give students, itself a function of teachers' knowledge of science content and learning goals.