This chapter explains the book’s aim and its approach to the history of the Irish nationalist youth organisation Na Fianna Éireann in the period 1909–23. It also provides a brief overview of the existing historiography on Na Fianna Éireann. It places the Fianna within the wider context of uniformed youth groups, which emerged in the late nineteenth and early twentieth centuries as a response to societal anxieties associated with the coming First World War. It compares and contrasts the Fianna to other youth groups in Ireland during the era of the Irish Revolution. These include the Boys’ Brigade, the Boy Scouts, the Hibernian Boys’ Brigade, the Clann na Gael Girl Scouts, the Irish Citizen Army Scout Corps and the Young Citizen Volunteers.