The idea of "threshold concepts" has been used to identify discipline-based concepts that are critical to that academic area. Threshold concepts are often difficult for students to assimilate in a meaningful way but, once done, can be powerful for the learner. In general, threshold concepts are 1) transformative to learner thinking; 2) bounded by the discipline; 3) integrative with other concepts; and 4) irreversible once understood (Meyer and Land 2003). This paper presents five threshold concepts in quantitative reasoning (QR) developed by transdisciplinary faculty workgroups that may be applicable for non-mathematics disciplines as well. They are as follows: 1) QR is an iterative process; 2) Abstract patterns can represent relationships between variables or objects; 3) There is a bidirectional translation between the concrete and the abstract; 4) Effective comparison depends on proportional reasoning; and 5) Different visual representations can communicate varying perspectives on the same quantitative information. The purpose of this paper is twofold: to explore and justify the proposed concepts as threshold concepts in QR-based courses and to offer practical examples which might assist students in developing and understanding these proposed threshold concepts.