The goal of this study is to investigate changes in PCK (Pedagogical Content Knowledge) of Dutch primary and secondary computer science teachers participating in a MOOC about Scratch programming. We captured the teachers’ PCK using identical pre- and post-questionnaires and conducted a qualitative deductive-inductive content analysis to identify changes in the PCK of the MOOC attendees. We relate the observed differences between PCK before and after the MOOC to Clarke and Hollingsworth’s model of teacher professional growth and Van Driel and Henze’s model of PCK development. Our analysis gives rise to four design principles meant to inform the pedagogical design of such MOOCs and improve their pedagogical affordances with regard to PCK development of their attendees.