The treatise "Contra legem Sarracenorum", written against Islam by the Florentine Dominican monk Ricoldus de Monte Crucis (? 1320), was widely used in the late Medieval and early modern period, and also translated several times. This thesis concentrates on the Greek translation made by Demetrios Kydones (ca. 1324?1397/8) and the Latin re-translation made by Bartholomaeus Picenus de Monte Arduo (fl. 1506), which soon outrivaled the original in terms of popularity, though ? or maybe because ? it somewhat differs in content. By comparing these three versions, the transformation processes, to which the text is subject, are elaborated and both the intention and the approach of the translators are investigated for the first time. It turns out that Demetrios Kydones and Bartholomaeus Picenus pursue completely different objectives and accordingly different translation methods, too. Whereas Demetrios Kydones takes into account both his target audience and the target language and thus provides an elegant rhetoric-communicative translation, Bartholomaeus works strictly literally-morphemically, even to the extent of disregarding target-language customs. Overall it can be stated that each translation renders the text in an even more polemical way, but softens indecent expressions and content which might be offensive to Christian readers. In addition to researching the particular case of ?Contra legem Sarracenorum?, the framing chapters give the broader context of Medieval and Humanist translation theory and practice.