Bos and Read present here two medieval treatises on concepts. These treatises were first unearthed by one of the editors in the course of a different project, namely the search for the origins of the notion of 'suppositio collectiva'. They appear to have attracted no attention since the middle of the fifteenth century. These are two of only three medieval treatises known to the editors explicitly devoted to discussion of concepts. That is not to deny that other works treat extensively of concepts among other matters. In the late fourteenth and fifteenth centuries it became increasingly common to devote single treatises to single matters-supposition, consequences, exponibles, obligations and so on. A more famous treatise on concepts is Peter of Ailly's Concepts, given a modern translation by Paul Spade. Peter's treatise was written in Paris in the early 1370s, and printed there and in Lyon several times in the 1490s. Thomas of Cleves' treatise was also written in Paris in the early 1370's, and that of Paul of Gelria some ten years later, if not in Paris then in Prague. Neither has been printed before. To preface the edition of the two texts, the editors provide an introduction discussing the origin of medieval conceptions of concepts and commenting in detail on the content of the two treatises. They also provide some biographical information on the authors and attempt to date and place their texts.