Clinical reasoning is a multi-faceted and complex construct, the understanding of which has emerged from multiple fields outside of healthcare literature, primarily the psychological and behavioural sciences. The application of clinical reasoning is central to the advanced non-medical practitioner (ANMP) role, as complex patient caseloads with undifferentiated and undiagnosed diseases are now a regular feature in healthcare practice. This article explores some of the key concepts and terminology that have evolved over the last four decades and have led to our modern day understanding of this topic. It also considers how clinical reasoning is vital for improving evidence-based diagnosis and subsequent effective care planning. A comprehensive guide to applying diagnostic reasoning on a body systems basis will be explored later in this series.