The evidence-based clinical guideline, SIGN 98 and its subsequent revised edition of SIGN 145, has been an internationally significant addition to our understanding of the autism spectrum disorders (ASD) evidence base and a useful ally for people with ASD, their parents, carers, and professional practitioners. This chapter explains the experience of a consultant child and adolescent psychiatrist, working in a relatively remote and professionally isolated part of Scotland, trying to understand and manage the clinical implications of the significant changes which occurred in our understanding of ASD during and beyond the 1990s. It recounts how his individual momentum to obtain the guidance of a valid and reliable evidence base for his patients dovetailed with a national impetus to achieve the world’s first national ASD guideline. It explains how this guideline then arose, the challenges of its development and the successful outcomes following its publication.