These lecture notes collect the material used in the advanced course 'Interactive Storytelling' organized biannually at the Department of Future Technologies, University of Turku, Finland. Its aim is to present the key concepts behind interactive digital storytelling (IDS) as well as to review proposed and existing IDS systems. The course focuses on the four partakers of IDS: the platform, the designer, the interactor, and the storyworld. When constructing a platform, the problem is to select an appropriate approach from tightly controlled to emergent storytelling. On this platform, the designer is then responsible for creating the content (e.g., characters, props, scenes and events) for the storyworld, which is then experienced and influenced by the interactor. The structure and relationships between these partakers is explained from a theoretical perspective as well as using existing IDS systems as examples.