Prior research has mainly focused on what makes professional development effective from the program design perspective. However, there is a lack of understanding about what teachers need for improvement in the context of educational reforms and curricular changes. This study used the pedagogical content knowledge framework to examine teachers' needs for professional development situated in specific science topics. Data were drawn from a total of 118 science teachers who participated in a professional development program over 3 years. First, this study identified a list of common science topics that teachers needed to improve in life science, physics science, and earth science. Also, teachers perceived the needs to improve teaching of certain topics for different reasons: themselves, students, and curricular changes. Moreover, data analysis showed that teachers needed improvement in multiple areas of pedagogical content knowledge: learners, instructional strategies, curriculum, and assessment. In particular, inquiry teaching was one of the greatest challenges for most teachers.