Perceived risk and related attitudes have been implicated as major factors in many of the difficult policy problems that face modern society (nuclear power, genetically modified food, etc). Experts often argue that no or very small risks are involved; people are still worried. Why? The standard answer is lack of trust. Data on trust and risk perception, however, point to only a weak relationship between the two (r approximately 0.3). It is suggested here that the reason for the surprisingly minor importance of trust is that people believe that there are clear limits to how much science and experts know. Results are presented from studies of risk perception of the public, experts, and politicians. Politicians and members of the public believe that there are many unknown effects of technology and such beliefs were strongly related to their perceived risk. Experts on nuclear waste, on the other hand, seemed to believe that little is unknown in their field of expertise. Regression analyses of risk perception showed the unknown-effects factor to be a more important explanatory factor than trust for the public and politicians.