This thesis examined clinical deterioration in the prehospital setting. As autonomous practitioners in a volatile and high-risk environment, the deteriorating patient can result in increased demands on paramedics. All too often patients are reported to have ‘suddenly’ deteriorated, when in fact physiological parameters demonstrate decline for a period of time. The deteriorating patient can be difficult to recognise. Therefore, defining prehospital clinical deterioration is fundamental to supporting clinical judgement and improving prompt recognition of clinical deterioration in this challenging setting.