Although clinical reasoning is both broad and complex, the term “clinical reasoning” is contested and multiple definitions have been mooted within different contexts. In its simplest form, clinical reasoning is regarded as a “decision-making” process. Other definitions outline it in terms of a complex cognitive process, posited within multiple contextual factors. Traditionally, clinical reasoning models have been based upon cognitive theories. More recently, interpretive theories have been applied. Despite extensive research over the past four decades, no consensus on how clinical reasoning actually occurs has been achieved. Accurate clinical reasoning is vital to patient safety. Its importance as an essential clinical competence for healthcare professionals is well established. Indeed, it is the crux of a clinician’s work. Frequently, Foundation doctors are the first to review acutely unwell patients. During out-of-hours shifts senior help can be scant and Foundation doctors may have to rely on their own initial clinical reasoning to manage acutely unwell patients. This PhD explores clinical reasoning development in the transition phase between final year medical student and Foundation doctor (5MB-FY1 transition) in relation to acutely unwell patients. It follows a cohort of final year medical students from a single UK university on their clinical reasoning journeys as they transition into Foundation doctors, focusing on the role of the simulated healthcare setting and the workplace. The principle research question for this PhD was how does clinical reasoning develop across the transition phase between final year of medical school and Foundation year one? Within this overarching research question, the following sub-questions were posed: What do participants understand by the term clinical reasoning? What types of clinical reasoning experiences do participants narrate? How do participants clinically reason for acutely unwell patients? Which factors do participants perceive as being facilitating and hindering to their clinical reasoning? How do participants’ clinical reasoning processes develop across the 5MB-FY1 transition phase? This PhD uses multiple methodologies derived from interpretive approaches in innovative ways to tap into clinical reasoning processes and its development across four data collection points: T1: group and individual interviews; T2: Ward Simulation Exercise observations and stimulated recall interviews; T3: workplace observations and stimulated recall interviews; T4: final interviews. Data were collected from T1/T2 and T3/T4 during the final year of medical school and Foundation year one respectively. Primary thematic analyses were carried out cross-sectionally and longitudinally in terms of what participants said and how they said it. Secondary narrative analyses were undertaken of participants’ Personal Incident Narratives. By taking an interpretive approach, the complexities of clinical reasoning processes, both in terms of internal cognition and external socio-cultural influences were illuminated, drawing upon clinical reasoning, complexity and situated learning theories. The key findings of this PhD were that participants conceptualised clinical reasoning as a “decision-making” and “thinking” process, leading to a clinical judgement for patient care; participants narratives aided understanding of clinical reasoning process and factors which facilitated and hindered them; participants used experiential knowledge and protocols to clinically reason for diagnosis, investigation, management and prioritisation; participants retained flexibility and contextual variability in the processes of making their clinical judgements; multiple factors facilitated and hindered the equilibrium of clinical judgement processes; and clinical reasoning development is dependent upon a complex interplay of individual, interpersonal and systemic factors which are deeply embedded in social-cultural theory. This study has multiple strengths and original features such the high participant retention rate throughout the longitudinal study, the exploration of the 5MB-FY1 transition, contemporaneous observations of clinical interactions with patients, the exploration of the out-of-hour setting contemporaneously and the multiple methods of data collection used in innovative ways. This PhD develops the published literature further in these domains. However, its challenges were predominantly ethical, such as lack of patient capacity to consent in the workplace.