Teachers often engage in curricular planning by critiquing and adapting existing curriculum materials to contextualize lessons and compensate for their deficiencies. Designing instruction for students is shaped by teachers' ability to apply a variety of personal resources, including their pedagogical content knowledge (PCK). This study investigated a criterion-based approach to lesson plan analysis as one way to help preservice elementary teachers develop and use their PCK to plan instruction for students. Results show that the preservice teachers demonstrated a range of strengths and weaknesses in applying their knowledge of science assessment, science curriculum materials, and instructional strategies for teaching science. This range was influenced, in part, by the presence of alternative ideas about science teaching, the extent to which the original curriculum materials aligned with reform-based standards and practices, and the presence of prompts to use criteria in their analyses. Despite these factors, preservice teachers' PCK improved significantly over time when they had multiple opportunities to practice applying the same criterion in their analyses. Insights into science teacher knowledge and implications for science teacher education are discussed. (Contains 4 tables.)