The present-day University Library of Amsterdam grew out of the city library which was founded in 1578 and based in the Nieuwe Kerk. In 1632 this same library became the library of the Athenaeum Illustre, the institute for higher education which was elevated to University status in 1877. This article discusses the first fifty years of the city library, the period from 1578-1632, in which the library was at the height of its glory before going into a sharp decline after the middle of the seventeenth century. The city library was founded in, or soon after, 1578 (the archives were destroyed in a fire), and was a result of the so-called 'Alteration', the transition of Amsterdam to the Reformation and the party of the revolt. The founders were the burgomasters of Amsterdam, whose object was to found a modern library of useful books on various subjects, freely accessible to all readers, who could choose their own reading matter. That this was the purpose of the library emerges from the papers of the famous burgomaster Cornelis Pietersz Hooft, the father of the poet. In accordance with this purpose only those books of the old parish library of the Nieuwe Kerk were taken over which were suited to a modern library. The rest were sold or exchanged for books and manuscripts which fitted a humanistic cultural programme. The city library was lodged in the premises of the Nieuwe Kerk and was run by the churchwardens. Hitherto scholars have searched in vain for the man responsible for assembling the library. This was probably Peter Vekemans van Meerhout, rector of one of the Latin schools in 1578. This Brabantine, from an ecclesiastical point of view regarded as a 'liberal', was known to be an expert on old manuscripts and chronicles. After his death in 1603 he was succeeded by the second librarian, the Englishman Matthew Slade, who had fled to Amsterdam as a follower of Robert Browne, but who later became a violent Calvinist under the protection of Petrus Plancius. Four years after his death in 1628 the city library was taken over by the Athenaeum Illustre and was transferred to the building of the Agnietenkapel. The layout of the library can be reconstructed from a printer's device of Lodewijck Spillebout, perhaps after a drawing by Willem Buytewech. The first catalogues appeared in 1612 and 1622, and are both the work of Slade. They contained 765 and 890 titles respectively, systematically arranged according to the bookcases, with an extensive author index. The 1612 catalogue, with an engraved title-page (probably after a sketch by David Vinckboons), was printed in Leiden and lacks a foreword. It was presumably a private edition printed thanks to a patron, possibly the Leiden scholar Petrus Scriverius. The printing costs of the 1622 catalogue were met by the churchwardens. The works purchased since the foundation of the library include some beautifully bound Greek manuscripts from the collection of cardinal Granvelle. Maybe also from the same collection is the famous manuscript with the Latin translation of Aristotle's Ethica, written in 1517 by the celebrated calligrapher Arrighi for Vittoria Colonna and illuminated by Attavanti. In the choice of printed books the emphasis lies on standard works, editions of texts, dictionaries, grammars and so on, Judging from various sources, the city library was much visited in this period not only by scholars, cartographers, etc., from Amsterdam and elsewhere, but also by younger men, like the poet Pieter Cornelisz Hooft.