Introduced by Meyer and Land in 2003, threshold concepts have since become a useful figure of speech for describing and explaining typical difficulties. Because of their reference to individual learning biographies of novices and experts, threshold concepts appeal to many instructors in the technical disciplines. However, in spite of this success, an unambiguous operational definition of threshold concepts in those fields has never been given. In this presentation we will show empirical data from introductory engineering education for which threshold concepts serve as a plausible theoretical model and suggest possible criteria for their identification. We also identify threshold concepts that are common to introductory courses in several engineering disciplines.