In order for intelligent systems to be applicable in a wide range of situations, end users must be able to modify their task descriptions. We introduce Tailor, a system that allows users to modify task information through instruction. In this approach, the user enters a short sentence to describe the desired change. The system maps the sentence into valid, plausible modifications and checks for unexpected side-effects they may have, working interactively with the user throughout the process. We conducted preliminary tests in which subjects used Tailor to make modifications to domains drawn from the eHow website, applying modifications posted by readers as ‘tips’. In this way the subjects acted as interpreters between Tailor and the human-generated descriptions of modifications. Almost all the subjects were able to make all modifications to the process descriptions with Tailor, indicating that the interpreter role is quite natural for users. Categories and Subject Descriptors I.2.6...