This article outlines the recently developed idea of "Threshold Concepts" and explains how this idea may be used to describe the progress of learning in economics. The authors examine the relationship between threshold concepts, key concepts and conceptual change and suggest that a distinction between basic, discipline and procedural concepts may be useful in determining a framework for the identification of threshold concepts. The current understanding of threshold concepts is extended by indicating that these might best be seen as a web of concepts which link thinking and practice in a discipline. A range of evidence is presented to illustrate the existence and implications of threshold concepts in economics. (Contains 3 tables and 2 figures.) [This project was also funded by the Department for Employment and Learning.]