There is a growing literature on how scientific experts understand risk of technology related to their disciplinary field. Previous research shows that experts have different understandings and perspectives depending on disciplinary culture, organizational affiliation, and how they more broadly look upon their role in society. From a practice-based perspective on risk management as a bottom-up activity embedded in work place routines and everyday interactions, we look, through an ethnographic lens, at the laboratory life of nanoscientists. In the USA and Sweden, (...) two categories of nanoscientists have been studied: upstream scientists who are mainly electrical and physical engineers and downstream scientists who are toxicologists, often with a more multidisciplinary background, including physics, chemistry, biology, and engineering. The results show that although the two groups of scientists share the same norms of appropriate laboratory conduct to promote safety and good science practice, they have very different perspectives on risk with nanomaterials. Upstream scientists downplay risk; they emphasize the innovative potential of the new materials to which they express an affectionate and personalized stance. The downstream scientists, instead, focus on the uncertainties and unpredictability of nanomaterials and they see some materials as potentially highly dangerous. The results highlight the ambiguous and complex role of scientific experts in policy processes about the risk and regulation of nanotechnology. (shrink)