Threshold Concept (TC) theory was introduced by Erik Meyer and Ray Land in 2003 and states that every discipline of study contains troublesome concepts that transform the learner when they come to be understood. This transformation will concern either the learner’s thinking about the discipline and/or their perception of the discipline. Five characteristics were originally proposed that can help to signalise TCs, which are, troublesome, transformative, irreversible, integrative and bounded. This thesis explores both, the in-depth ways in which TCs have sought to be identified, and to what extent the role of TCs in curriculum development has been studied. This was done by exploring researchers’ thinking in regard to the importance of each of the five TC characteristics, the data collection methods and participants that were included in their studies, and the extent to which they attempted to study TCs in relation to the development of curricula. Engineering education TC studies formed the context through which I attempted to answer the research questions. The first method I used to obtain my data was to carry out a systematic quantitative literature review (SQLR) of peer-reviewed TC research articles that were published between 2003 and the present day. From the relevant 147 papers that resulted from my database searching, I narrowed this down to the 15 articles that related directly to engineering education. The SQLR data was then used to inform the semi-structured interviews that I conducted with three TC researchers in the field of engineering education. The findings suggested troublesome and transformative to be the TC characteristics that are the most essential of the five to consider when seeking to identify TCs. This finding was based on the fact that they were the two characteristics most highly referenced in the SQLR literature and were the ones thought most important by two of three of my interviewees. The inclusion of multiple types of participants and data collection methods were found to be common and effective practices amongst TC researchers. Again, this finding was supported with the SQLR data and by all three interviewees. Lastly, it was discovered that, despite TCs being considered the potential centrepieces of a curriculum, there is a lack of TC studies focussing on the role of TCs in curriculum development. There existed no studies within the SQLR literature that held this aspect as a sole focus.