After outlining current approaches to immersive theatre, Biggin introduces her status as an embedded researcher with immersive theatre company Punchdrunk and proposes new theoretical approaches to immersive audience experience. This introductory chapter considers contemporary approaches to "the audience" in theatre, film and cultural studies, then redefines "immersion" with a model drawn from cognitive science, philosophical aesthetics and the study of computer games. Biggin introduces new concepts for theatrical immersion including graded states, barriers to immersion and Real World Disassociation; and proposes a distinction between "immersive theatre" the form and "immersive experience" the audience sensation. The chapter concludes by outlining the book, which uses Punchdrunk productions to explore immersive audience experience in relation to interactivity and game, story and narrative, and environment and space.