This book is about the participatory aesthetics of the fantastic. In it, the author argues that the definition of the fantastic presented by Tzvetan Todorov in 1970 can be used, provided it is first adapted to a media-ecological framework, to theorize the role of aesthetic participation in the creation of secondary worlds. Working within a hermeneutical tradition, Todorov understands reader participation as interpretation, in which the creative ambiguities of the literary object are primarily epistemological. However, it is here argued that the aesthetic object of the fantastic is also characterized by material ambiguity. The purpose of this dissertation is then to present a conceptual framework with which to theorize the relation between the material and the epistemological ambiguity of the fantastic. It is argued that such a framework can be found in an ecological understanding of aesthetic participation. This, in turn, entails understanding human subjectivity as a process always already embodied in a material environment. To this extent, the proposed theoretical framework questions the clear and oppositional distinction between form and matter, as well as that between mind and body, nature and culture, and human and non-human, on which a modern and humanist notion of subjectivity is based. And in this sense, the basic ecological assumptions of this dissertation are posthumanist, or non-humanist. From this position, it is argued that an ecological understanding of participation offers a means to reformulate the function of a number of concepts central to studying the aesthetics of the fantastic, most notably the concepts of media, genre and text. As the fantastic focuses on the creation of other worlds, it is an aesthetics of coming into being, of ontogenesis. Accordingly, it will be argued that the participatory aesthetics of the fantastic operationalizes the ontogenesis of media, genres and texts. By mapping the ontogenesis of three distinct media ecologies – the media ecology of fantasy and J. R. R. Tolkien’s secondary world Middle-earth; the media ecology of the American comic book superhero Miracleman; and the media ecology of William Blake – this book argues that the ecological imagination generates world. Per Israelson has been a doctoral candidate in the Research School of Studies in Cultural History at the department of Culture and Aesthetics, Stockholm University. Ecologies of the Imagination is his dissertation.