As the boundaries of the Requirements Engineering (RE) discipline keep expanding, deciding on which contents a course or module should include has become challenging. In absence of an agreed-upon syllabus for RE higher education, we take a step back and conduct a study that aims at identifying threshold concepts: a subset of the core RE concepts that, when mastered, are expected to let the learner make a leap in the understanding of the discipline. Through a two-phase mixed-methods design, consisting of individual semi-structured interviews followed by two focus groups to achieve consensus, we identified two threshold concepts as well as seven candidate ones. The two threshold concepts pertain to pillars of the RE discipline: understanding that RE is a co-creation process between analysts and stakeholders, and the importance of eliciting goals for uncovering the stakeholders’ true needs.