This article extends the notion of "knowledge at the mathematical horizon" or "horizon knowledge" introduced by Ball and colleagues as a part of teachers' subject matter knowledge. Our focus is on teachers' mathematical knowledge beyond the school curriculum, that is, on mathematics learnt during undergraduate college or university studies. We explore and exemplify what horizon knowledge may contribute to teaching elementary and secondary school mathematics in terms of relevant subject matter, as well as how such knowledge can shape interaction with students. We introduce a new perspective to the on-going discussions around mathematics for teaching by considering what may lie at the horizon of teachers' mathematical knowledge.