This study reports on the initial work on the use of Threshold Concept Theory (TCT) to develop a threshold-concept inventory – a catalogue of the important concepts that underlie electronics and electrical engineering (EE) – and an assessment tool – to investigate the depth of student understanding of threshold and related concepts, independent of students’ numerical ability and knowledge mimicry in the first-year course in electrical engineering. This is both challenging and important for several reasons: there is a known issue with student retention (Tsividis, 1998; 2009); the discipline is relatively hard for students because it concerns invisible phenomena; and finally it is one that demands deep understanding from the very start (Scott, Harlow, Peter, and Cowie, 2010). Although the focus of this research was on electronic circuits, findings regarding teaching and learning of threshold concepts (TCs) will inform lecturers in three other disciplines who are part of our project on threshold concepts.