RATH is an adaptive tutoringWWW software prototype combining a mathematical model for the structure of hypertext with the theory of knowledge spaces from mathematical psychology. Using prerequisite relationships between di erent items in a domain of knowledge and using the knowledge about the pupil's current knowledge state RATH presents only those links in a hypertext document to the pupil which point to a document for which he/she ful ls all prerequisites and which he/she therefore should be able to understand. In a rst prototype course, this idea is applied to the eld of elementary probability theory. This tiny course is based on prior research of the second author. Thus, RATH is an important step in bringing psychological theories into a working tutorial system.