A threshold concept is something that, when learned, opens up a new way of thinking about a problem, a subject or the world. An example is the physics concept of ‘heat transfer’ that can inform everyday activities, such as cooking or home energy use. These concepts help to define subjects, and they shift learners’ perceptions of a topic area. Teachers may use threshold concepts as starting points for the design of effective lessons. The idea of threshold concepts can itself be difficult to understand. How to identify them and distinguish them from other learning topics has provoked debate among academics. Classroom education is designed to transform learners so they gain a deep knowledge of the concepts needed to understand and apply a curriculum topic and can fit these concepts together appropriately and accurately. Threshold concepts can provide a starting point for this transformation.