Beginning with the origins of conscientious objection, the chapter considers the background to this concept, focusing on: its origins, particularly its relationship to ‘religion’, ‘belief’ and matters of ‘conscience’; the relevance of ‘State neutrality’, its relationship with cultural heritage and the significance of secularism. It examines the meaning of ‘conscientious objection’ and how this relates to rights of autonomy; it analyses “conscience”, exploring its roots in ethics. It considers: the role of the ‘conscientious objector’, examining what distinguishes it from that of a ‘whistleblower’ or social activist; the relationship between authority and the right to object; the distinctive characteristics of ‘conscientiousness’ and how this is interpreted in law. The chapter then turns to examine the evolving policy governing the matters to which an objector has been able to claim legal exemption on the basis of religion/belief/conscience. A brief historical narrative provides an overview that tracks the role of the principle from early application in relation to the taking of oaths and armed combat to an evolving policy regarding, successively, matters such as vaccinations, abortion, LGBT and transgender issues, IVF, genetic engineering, stem cell research and medically assisted dying. This exploration of the many aspects of conscientiousness prepares the way for the examination of objection and dissent that follows in the next chapter.