This research seeks to identify the knowledge, skills, personal qualities and understanding underpinning expert practice in response to emergency calls, the core role of the paramedic, and to explore how paramedics believe their expertise was a role I developed. In-depth interviews grounded in recalled incidents (calls) were employed to generate data from which deductions were made about the underlying capabilities and guiding principles of the work of paramedics. This was supplemented by a questionnaire and the data analysed using a range of theoretical lenses. Responding to a call is described through a sequential series of key activities that, in practice, merge into each other, often in an iterative way: information gathering, managing situations and people and treating patients. Such a description leads to an identification of expertise used in the early stages of responding to a call, such as focusing on reducing and managing ambiguity through the utilisation of situated knowledge to generate tentative hypotheses about the nature of the call and the development of initial plans for action. Such hypotheses are left open and modifiable in the light of new information actively sought by the expert practitioner and are guided by capabilities such as: communication, planning and organising, decision making / problem solving and learning from experience. Additionally, expert paramedic practice is characterised by high levels of resilience and flexibility needed to leave plans incomplete for further development in the heat of practice. This thesis therefore characterises the expertise underpinning the work of experienced paramedics in a way that transcends attempts to describe paramedic practice through lists of skills, knowledge or competencies to be acquired. In so doing the thesis contributes to the evidence base about the knowledge and skill used by paramedics in practice, and how and where this is developed.