Technological developments in the field of nanoscience and nanotechnology have led to the development of several newly engineered nanoparticles. These materials are already being applied in a variety of consumer contexts, as well as business products, and are expected to be more widely used in the coming years. Despite all the promises, novel nanoparticles are still accompanied by scientific uncertainty about their hazardous effects. Due to these uncertainties, conventional methods for managing risk are deemed insufficient. In response to this, it has been proposed that the management of uncertain risks requires a forwardlooking notion of responsibility, which entails various kinds of anticipatory activities to ensure an adequate and timely response to emerging risks. The question of who should bear this responsibility often remains implicit in such discussions. However, innovation processes cannot be responsible, nor can they reflect on or account for what they do, or make intentional choices. Ultimately, responsibility should rest with particular individuals. At the same time, it has been recognized that the collaborative nature of innovation processes creates problems for the allocation of responsibility to individual actors such as scientists, engineers and product developers. Activities that lead to the development of nanoparticles and nanoproducts are often complex and distributed; they take place at multiple locations, combine insights from several disciplines and involve many different agents. This suggests that in order to determine a viable allocation of responsibility for uncertain nanoparticle risks, it is necessary to reflect on the way people interact in the field, and how this influences the capacity of individuals to take responsibility for emerging hazards. This thesis contributes to this discussion by exploring the relationships between people who are involved in various ways in the development and use of nanoparticles, and by exploring how such relationships should be taken into account in the allocation of responsibility. The ultimate aim is to align the responsibilities that people have with the relevant relationships in the field. The goal of the thesis is primarily normative: it develops a framework to ethically assess relationships. However, the normative analysis is strongly empirically informed: it is supported by data from two case studies, one on the use of nanoparticles in a work environment and one on the use of nanoparticles for Managing the uncertain risks of nanoparticles 162 land remediation, complemented with literature from the empirical sciences and a collaborative paper with two nano-engineers. As a first step, this thesis explores how relationships matter to the way we deal with the uncertain risks of nanoparticles. Exploring the possibility of informed consent in relation to nanoparticles, Chapter 2 shows that several features of relationships, such as dependency, proximity and the existence of shared interest can influence the quality of decision-making processes about uncertain risks. Following this, Chapter 3 shows that relationships, such as those between employer and employees, can give rise to duties of care for uncertain risks. Chapter 4 argues that the existence of relationships is necessary in order to be able to respond to new and emerging hazards in the field of nanoscience and nanotechnology. On this basis, Chapter 4 argues that in some cases there is an obligation to establish relationships. In particular cases, where there is a need for collective action, but no such collective exists, individual engineers involved in innovation processes would have a duty to collectivize: they must organize themselves into a collective that can adequately act upon emerging and unwanted hazards. Finally, in Chapter 5, this thesis explores the characteristics required of such relationships to foster responsibility in nanoparticle development. In doing so, I shift from a narrow notion of responsibility focused on dealing with risks to a broader conception of responsibility that not only takes into account risks, but includes the potential benefits of innovation as well. The chapter develops a framework to characterize morally relevant features of relationships based on the Ethics of Care. Several features of relationships are identified that can be used to evaluate whether relationships amongst those developing and using nanoparticles are caring. These include dependency, power, attention, responsiveness, emotional engagement and availability. The usability of this framework is explored by applying it to the context of innovation in relation to nanoparticles used in the context of land and water remediation. In Chapter 6, the thesis concludes with a reflection on the alignment of relationships and responsibilities, discussing whether relationships should be adjusted to responsibilities or – vice versa – whether the responsibilities we allocate for nanoparticle risks should be adjusted to the relationships at hand. I suggest that there is a middle ground between these two options – an understanding of responsibility that is based on a certain characterization of relationships. This understanding holds that having ‘the right kind of relationships’ is part of what it means to take responsibility. This thesis ends with a discussion of the implications of these findings for the practice of and scholarship in Responsible Research and Innovation.