Intelligent Tutoring Systems authoring tools are highly complex educational software applications used to produce highly complex software applications (i.e. ITSs). How should our assumptions about the target users (authors) impact the design of authoring tools? In this article I first reflect on the factors leading to my original 1999 article on the state of the art in ITS authoring tools and consider some challenges facing authoring tool researchers today. Then, in the bulk of the paper, I propose some principled foundations for future authoring tool design, focusing on operationalizing the construct of complexity—for tool, task, and user. ITS authoring tools are major undertakings and to redeem this investment it is important to anticipate actual user needs and capacities. I propose that one way to do this is to match the complexity of tool design to the complexity of authoring tasks and the complexity capacity of users and user communities. Doing so entails estimating the complexity of the mental models that a user is expected to build in order to use a tool as intended. The goal is not so much to support the design of more powerful authoring tools as it is to design tools that meet the needs of realistic user audiences. This paper presents some exploratory ideas on how to operationalize the concept of complexity for tool, task, and user. The paper draws from the following theories and frameworks to weave this narrative: Complexity Science, Activity Theory, Epistemic Forms and Games, and adult cognitive developmental theory (Hierarchical Complexity Theory). This exploration of usability and complexity is applicable to the design of any type of complex authoring application, though the application area that motivated the exploration is ITS authoring.