International interest is growing in how threshold concept theory can transform tertiary teaching and learning. A facilitated practitioner action research project investigating the potential of threshold concepts across several disciplines offers a practical contribution and helps to consolidate this international field of research. In this article we show how a focus on threshold concept theory enabled tertiary teachers to work collaboratively to investigate tertiary pedagogical practices. The purpose of the article is to argue that threshold concept theory can serve as a guiding principle of pedagogical design. The article draws on findings from a research study conducted over two years by a team consisting of five practitioner researchers in four disciplines and two educational researchers who facilitated the inquiry. The act of constraining the research to thresholds, both in and across different fields, enabled the team to intensify discipline-specific insights and to explore wider cross-disciplinary links and differences. A threshold-constrained focus entailed making specific discipline, knowledge management, and pedagogic practices explicit to ourselves as individual practitioners and comprehensible enough to enable conversations with colleagues from other disciplines. As a result of the research, we argue that threshold-concept thinking enables three processes: usefully unsettling the meaning of being a disciplinary expert; providing a structured framework for both disciplinary and cross-disciplinary knowledge and learning; and intensifying insight into curricular content and teaching methods. We also provide an account of how the collaborative action research sparked fresh experiments, searches for new data, and reflections on the impact of threshold concepts on individual disciplines and beyond.