Interactive digital storytelling (IDS) allows a human user to become an active part in a story and to affect how the story unfolds. To understand IDS systems, we need to consider the partakers present in them as well as their roles and interconnections. In this chapter, the authors discern four partaking entities—interactor, author, developer, and storyworld—and describe both their affiliated sub-entities as well as their relationship to one another. Based on both reviewing relevant literature and analyzing existing IDS systems, the ontology presented here provides a cohesive view into the current state of both theoretical and practical research.