Teachers are increasingly being called upon to teach in ways that develop 21st century learning skills in their students. Various frameworks for 21st century learning have been proposed and while they differ, all agree on four components for development--creativity, collaboration, communication and critical thinking. Both individually and together, STEM subject areas contribute to the development and enactment of these skills through inquiry-based approaches to learning. Although integrated approaches to teaching the STEM disciplines afford enhanced opportunities to develop these skills, they rely on teachers having expertise in at least one and ideally more than one of the relevant underpinning disciplines. At a time when many countries are experiencing shortages of adequately qualified teachers of mathematics and some science disciplines, this presents an especially difficult challenge. Similarly, if teachers are to facilitate their students' 21st century competence they need to have this competence themselves--a fact that appears to have been largely ignored to date. In this paper we present a framework that enables novice teachers (novice to teaching in general, teaching a STEM discipline, or teaching integrated STEM) to think in detail about what they need to know, find out, or think about as they plan for teaching, enact teaching, and reflect on teaching. As well as explicating the complexity of the knowledge of teachers of individual and integrated STEM disciplines, the framework highlights the importance of teachers' own 21st century skills. Finally, we suggest ways in which teachers might use or adapt the framework to assist their students to develop their own 21st century competence.