Electronics and circuit theory are acknowledged as troublesome subjects when first introduced to students. This leads to low student retention into later electronics courses, especially in universities that offer a common first year where students are free to change streams after the first year. We report on a detailed study of the application of Threshold Concept Theory to an introductory electronics course. We identify some Threshold Concepts, explicit and tacit. We postulate that a high density of Threshold Concepts accounts for the reputation for troublesome learning in, and low retention following, these courses. We further suggest that the bimodal distribution of marks that is commonly observed in electronics teaching is a hallmark of a Thresold Concept. This may have significant impact on assessment.