The introduction to this study provides a sketch of the interlinked stories of the standardisation of psychological diagnostic categories and asylum medical statistics that occurred in the British Isles between the mid-nineteenth century and the end of the Second World War. The history presented in this volume has four main sections: the first provides an assessment of the conditions that allowed the Table of the Forms of Insanity to come into being; the second provides an outline of the first two sets of debates on standardisation of diagnostic terms in asylum admissions procedures and statistics; the third section addresses mental health statistics during the first half of the twentieth century; and the fourth and final section outlines the circumstances surrounding the replacement of the Table of the Forms with the World Health Organisation’s International Classification of Diseases in 1948. This introduction outlines the goals of the long durée history to explore the links between governance and the development of clinical classifications of mental disorder that shaped early mental health statistics. The history of the Table of the Forms and the debates that shaped the classification took place within the wider context of asylum administration and the collection of statistical knowledge on psychological disorders.