This paper presents the EarthTutor authoring tool, a multi-layered system designed to remove the technical hurdles preventing teachers from having full control over the structure of their lessons, without sacrificing the power and technical flexibility required for an effective ITS implementation. EarthTutor reduces the ITS design problem to a series of related but independent layers. Each layer in the process hides the implementation details of lower layers, while providing a foundation for higher levels. This approach allows a core group of advanced authors to do the more complex authoring tasks and export their work to a wider audience of novice authors, who complete the authoring process.