Science communication and science education have the same overarching aim--to engage their audiences in science--and both disciplines face similar challenges in achieving this aim. Knowing how to effectively engage their "audiences" is fundamental to the success of both. Both disciplines have well-developed research fields identifying best practice. However, there seems to be an impediment in putting this knowledge into practice, or even much sharing of knowledge between the 2 disciplines. Threshold concepts refer to concepts that are fundamental to thinking and practice in a discipline. In this paper, we argue that engagement is a threshold concept for both science education and science communication. Considering the vast amount of literature on science education and science communication, the focus in this paper is on recent recommendations rather than longstanding, more general notions, providing a contemporary view. This paper illustrates how engagement fulfils the characteristics of a threshold concept for both disciplines and highlights how the 2 fields could assist each other. The purpose of this paper is to spark new conversations and sharing between the 2 disciplines, with the use of threshold concepts as a vehicle for enabling collaboration and progress.