CAT0 is an instructions environment for practicing basic skills of legal research: to use cases in arguments about a problem situation and to test a theory about a legal domain. Using the CAT0 tools, law students analyze a legal problem, frame queries of CATO'S database of legal cases, and judge how relevant the retrieved cases are to their developing argument or theory. CAT0 aids hming by making explicit an abstract model of the process of argument. It allows students to focus on the high-level argumentation issues, by assisting the student in various ways. By providing an abstract representation of the text of cases, it helps students to reason about the texts and helps guide their critical analysis of the texts. CAT0 makes available opportunities for practice that are hard to set up with traditional instructional methods. CAT0 differs from other instructional environments in the following respects: Few instructional environments focus on argumentation skills. Although there are other instructional environments in which students work with an abstract representation of the task domain, abstracting from text is unusual. CATO demonstrates a contribution that case-based reasoning techniques can make to instructional environments.