Although the Isagoge features in the Latin tradition from soon after it was written until 1200 (and, of course, beyond), the quantity and richness of the evidence for its use and interpretation in the twelfth century far outweighs that from the preceding eight hundred years. Most of this evidence consists of commentaries, usually anonymous and largely unpublished. They are catalogued in a substantial appendix to this article. The catalogue is not an entirely new one – I published a first version of it 25 years ago. But the research since this time – above all, that carried out and generously made available to his colleagues by Yukio Iwakuma – has so transformed our knowledge of the material that much of the significant information, about dates, authorship and affiliation has been changed or augmented. The second, longer, part of this article introduces this material and the recent research on it, and the picture that emerges of how the Isagoge was studied at the time. The first part looks more rapidly at the place of the Isagoge in the Latin tradition in the preceding centuries – a topic that has not before received individual treatment.