It is argued that the social significance of nanotechnologies should be understood in terms of the politics and ethics of uncertainty. This means that the uncertainties surrounding the present and future development of nanotechnologies should not be interpreted, first and foremost, in terms of concepts of risk. It is argued that risk, as a way of managing uncertain futures, has a particular historical genealogy, and as such implies a specific politics and ethics. It is proposed, instead, that the concepts of contingency and of finitude must be central to any understanding of the ethical significance of nanotechnologies, as these concepts can be used to understand the basis of recent work in science and technology studies, and the sociology of knowledge more widely, which details the multi-dimensional social nature of technological uncertainty.