In the twelfth century, logic was studied on the basis of a handful of textbooks and commentaries, mainly by Boethius, Aristotle and Porphyry. The first part of our chapter provides a statistical study of the manuscript distribution of this material, showing which texts were the most popular, and which combinations of material were most common. It also tracks how previously unknown Aristotelian logical texts (the logica nova) were introduced into the curriculum. The second part studies the manuscripts of original logical works composed in the twelfth century. Whether they are treatises or commentaries, they are usually found in only one or two manuscripts. The commentaries, however, show a pattern of complex interrelations with one another: different commentaries in different manuscripts may well be based, at least in part, on the same lectures. Some manuscripts, which are studied in detail, bring together a number of these anonymous, fluid and often multi-layered commentaries.