As recent reflections on posthumanism, in part orchestrated by a conference session on 'Post-human/Post-natural Geographies' from which these papers emerge, have suggested, the term posthumanism stands for a series of quite different perspectives and positions. Posthumanism is used to describe a historical condition and to signal a theoretical perspective. These different uses of the term reflect often radically different and opposed approaches to the category of the 'human'. On the one hand, posthumanism names a contemporary context in which new scientific developments trouble the foundational figure of the human subject as distinct from other animal forms of life. New biotechnologies which involve the transfers of substance (genes or organs) between species and new distributions of abilities, alliances and dependences between people and machines disturb an idealized definition of the human subject as separate and liberated from nature and fully in command of self and non-human others. Accounts of a posthuman present or imminent posthuman future may be framed by either an anti-humanist celebration of this shift or by humanist concern about the apparent fracturing of the human subject. However, anxieties about the erosion of the ideal human subject differentiated from and in power over the non-human world of 'nature' suggest a continued investment in a model of the modern human that has been the subject of significant critical attention. There is a considerable body of work within feminist and other critical traditions that addressed the ways the category of humanity as separate from and in control over nature has historically also involved hierarchical models of difference amongst humans (Braun 2004; Murdoch 2004). Humanism's history is a history of race and sex being used to define some humans as more human through their distance from nature than others (Anderson 2001, 2003; Grosz 1993).