Research and design activities are becoming more important in Science, Technology, Engineering and Mathematics (STEM) and D&T (design and technology) education. Research and design are often taught separately from each other, while in professional STEM practices, many projects are neither 'research only' or 'design only'--they are both. In this study, we aimed to provide insights in teachers' personal and shared knowledge on how research and design can be connected. To this end, we examined the development of pedagogical content knowledge (PCK) and beliefs of six teachers of the Dutch STEM subject O&O (research and design), who participated in a professional learning community (PLC) aimed at connecting research and design within this subject. Results of pre and post-PLC interviews showed that teachers' personal PCK was very diverse, probably due to their different beliefs, backgrounds and teaching contexts. Through jointly designing instructional strategies for connecting research and design, teachers contributed to a collective knowledge base. The results of this study indicate that a professional learning community in which teachers with varying backgrounds construct knowledge and instructional strategies together, can be a powerful method to enhance personal PCK and collective knowledge. These are promising outcomes in the light of shaping professional development activities for STEM and D&T teachers, which in turn aims to provide students with a holistic and realistic view on current professional STEM fields.