A learner-centered metathesaurus will be created by studying the transactions between learners and two learning resources - the Math Forum (mathforum.org) and www.smete.org. Three modes of interaction will be studied. Data in which many students respond to the same problem (the MathForum Problem of the Week) will be used to establish conceptual structures within each of six problem domains. The large number of participants (500-1000 per question, several hundred questions) will provide many modes of math communication. Data from transactions between learners and experts (the MathForum Ask Dr. Math) will be used to relate these conceptual structures to the generic domain structure used to index the transaction archive. The resulting set of related knowledge structures will provide the basis for learners to navigate the space of math resources, from static instructional units, to group projects, to small group discussions, etc. An additional resource, www.smete.org will be studied. Here, learner interaction is more in the typical library mode of search-retrieve. Because the library is digital, we have additional feedback information - viewed and selected resources. This will allow us to map between learner language and resource metadata, extending the metathesaurus across domain boundaries. The result of the project will be to provide a learning emphasis for the SMETE Digital Library and a set of tools for creating inter- and cross-domain thesauri.