During the past decade, early childhood teachers have been faced with new needs to develop their content knowledge and pedagogical content knowledge (PCK) for different science subject areas. In order to meet these challenges there is a strong need for professional learning programmes for early childhood teachers that focus on the development of knowledge, and skills to work with science activities in their own context. In this article, 46 early childhood teachers participated in a professional development programme aimed at helping teachers capture and further develop their content knowledge and PCK for teaching science through the use of content representations (CoRes). A CoRe is a holistic reflective tool for making explicit the different dimensions of, and links between, knowledge of content, teaching, and learning about a particular topic. The aim of the project was to investigate in what ways the use of the CoRe helped the teachers in planning and reflecting on their science teaching and in developing their PCK. Thus, the project was based on opportunities for teachers to develop and reframe their knowledge of science teaching in ways that stimulated children's curiosity for scientific phenomena. The result indicates that with its content-specific focus, the CoRe helped the teachers to highlight the science content in their activities, not only the general pedagogical knowledge. The use of the CoRes also helped the preschool teachers to establish the fundamental ideas of the topic they were teaching and to develop confidence in what they were teaching, why, and how.