This chapter argues that speculative fiction is an important tool for the posthumanism project of deconstructing and critiquing the default “man” of western humanism. The genre’s techniques of defamiliarization and literalizing metphor find parallels in works of posthumanist theory, and I argue that this fiction is a form of everyday theorizing of the same questions that inform posthumanist theory. Like posthumanist theory, science fiction questions what it means to be human, and often attitudes agency and cognition to nonhuman entities. This chapter reviews the genre’s contributions to posthumanism in the areas of transhumanism and smart systems, genetic engineering and synthetic biology, the Internet of Things, and climate change and ecological sustainability. It argues that there is a reciprocal exchange between imaginative visions and material practice, making sf a discourse of world transformation.