What is the role of objects in political participation? This question is today being asked by researchers, theorists and practitioners of public engagement working in a range of different areas. Those concerned with environmental issues are looking for ways to account for the capacities of non-human entities — like trees and nuclear power plants — to mobilize publics, while in the world of design and innovation the crafting and handling of things are suggestive of new, inventive techniques for organizing political and ethical collectives. There is, then, a broadly shared commitment to accord to material entities the capacity to inspire, disturb, provoke and surprise in politically and morally significant ways. But, remarkably, the categories of political and social theory themselves have proven quite impervious to these effects. In these fields, the temptation has been strong to approach the inclusion of non-humans in democracy as a project of ‘extension’: engagement with the issue of non-human entities here all too often concentrates on the question of whether the existing machinery of politics, morality and ethics can be extended to include these entities. In this book I take a different approach, as I examine how the project of ‘letting things in’ transforms a specific category of social and political life, that of participation. I propose to understand material forms of engagement as a particular modality of participation, one that can be distinguished from and compared to other forms, and which brings with it particular problems, aspirations and indeed ideals.