In exploring the learning experiences of tertiary students, some educationalists have advanced the ‘threshold concept hypothesis’ according to which certain concepts in various disciplines act as thresholds. Such concepts need to be mastered before further progress can be made in a discipline – they are thus like portals or entrances to be traversed before students can think like practitioners of that discipline. In economics, the concept of opportunity cost has been selected as a prime example of a threshold concept. This paper subjects the threshold concept hypothesis to critical scrutiny on logical and methodological grounds, and then investigates its applicability to the economic concept of opportunity cost. The main conclusions are that the hypothesis has serious definitional problems, construing opportunity cost as a threshold concept is difficult, and the hypothesis is enmeshed in problematic conceptual issues.