This paper considers how the notion of pedagogical content knowledge (PCK) has developed and changed over the past 20 years. It draws on the preceding papers in this special issue to reflect on the big picture issues and ideas that are important in shaping understandings of PCK and the nature of research on PCK. The paper is structured around four sections. The first section offers analysis of the characteristics of PCK as employed by the researchers in this volume, raising some issues related to the construct. The second section considers the implications for research design that result from these authors' views of PCK. The third section suggests an agenda for future research on science teacher knowledge. The concluding section examines the continuing value of the PCK construct for the science education community.