Most 16-century prints from the “Hosianum” Library in Olsztyn are theological works, both Catholic and Lutheran. They include works by well-known reformation writers, such as Erazmus of Rotterdam, Martin Luther, Philip Melanchthon, Jean Calvin, Johannes Bugenhagen, as well as less known authors. The counter-reformation movement is represented in the library by the works of John Fisher, Johann Eck, Stanislaus Hosius, Melchior Cano and Robert Bellarmine. This article is not a complete catalogue of reformation and counter-reformation prints kept in the collection of the library. It names and discusses the most typical works from that period, as well as – according to the authors – the most interesting ones.