This chapter pursues an argument that librarianship has a long tradition of consistent ethical statements regarding the profession that, in more or less formal ways, have been prescriptive in character. The main focus is the relation between these statements and the kinds of documents in which they are formulated. It is argued that at various times in the history of librarianship certain types of documents have rendered ethical aspects of the library profession legitimacy and thus have taken on a prescriptive role. Empirically, three such documents are analysed; an ethical code from the early 20th century, a 17th century handbook in librarianship and a Papal Bulla from the late 15th century. As the main focus of the article lies on the long period before the emergence of formal ethical codes, emphasis lies on the two latter documents. Alhough different in character and scope these documents construct a continuous ethical foundation for librarianship in at least three aspects: (1) definitions of professional identity, (2) statements of core values, and (3) reiterations of multiple obligations. Seen in relation to historical periods and types of libraries—Bibliotheca Apostolica in Rome, a baroque library in Paris, and American public libraries—these documents create legitimacy for the profession through their respective contemporary relevance. This, in turn, raises the question of which kinds of documents are to formulate legitimate ethical guidlines for librarianship in a future where relative authority and legitimacy of different types of documents are becoming increasingly dificult to discern.