As we have developed a number of authoring systems for simulation-centered tutorial development, each successive authoring system has achieved its original goals, and yet has been quickly shown to have limitations that motivated the next authoring system. The history of these systems reveals two emerging requirement themes. The first theme is that there is a need for more than one level of tutor development. For example, a two-level authoring system might provide an easy-to-use system tbr novice tutor developers and a deeper level of authoring for expert tutor developers. A second theme is that the tutor delivery system should be componential and open so that it can work with collaborating applications, some of which may not have even existed when the authoring system was originally designed. The second part of this paper describes one essential element of a componential approach to simulation-centered tutor development, the specification of simulation services that may be required by tutors.