Nanotechnology is seen as a transformative technology, which has the potential to stimulate scientific innovation while greatly benefiting society. However, the enthusiasm with which the scientific and technical communities are embracing the technology is being tempered by concerns over possible downsides, including risks to human health. "Are these concerns valid?" is a question being asked by many, but frequently from differing perspectives. Given the increasingly complex interface between nanotechnology and society, relevant answers will be built on solid science and framed within a societal context.