In Chapter 1, we learnt about the diverse conceptions of integrated STEM held by teachers and appreciated the fact that integrated STEM inquiry shares similar epistemic practices (such as formulation of questions, logical deduction, and testing of ideas) with science and mathematical inquiry. However, integrated STEM inquiry differs from science and mathematical inquiry with regard to the object of inquiry—real-world problems that require the application of practical and disciplinary knowledge to solve them. While we acknowledge and celebrate the variations in teachers’ conception of integrated STEM inquiry, highlighting the similarities in ideas and practices is important for the community to develop common understanding of integrated STEM inquiry. In Chapter 2, we described the planning of integrated STEM inquiry using the STEM Quartet instructional framework. Whether we choose a problem, solution, or the user as the starting point, the learning experiences planned take into consideration how specific disciplinary knowledge is connected to one another. In this chapter, we will delve into the disciplinarity of integrated STEM inquiry to distil the defining characteristics of integrated STEM inquiry and its enactment. Understanding the disciplinarity associated with integrated STEM inquiry will enable teachers to reflect on their professional actions and decisions. The questions that we seek to answer are ‘What are the defining disciplinary features of integrated STEM inquiry?’ and ‘What are the different ways to reflect on enactment of integrated STEM inquiry?’