Through the mass media we frequently encounter experts who analyse the current economic situation, give advice on health problems, warn against global warming or comment on the chances and risks of technologies. These experts are affiliated with government agencies, companies, hospitals, NGOs, universties or other scientific organisations. This chapter focuses on scientists as public experts and explores their role when they not only talk about their research in public but use their special knowledge to provide orientation and advice relevant to individual or political problems of a lay audience.1 The expert role is particularly challenging for scientists because most are primarily concerned with the creation of knowledge, not its practical application, and scientific knowledge is usually insufficient as the sole base for giving advice. Furthermore, as information sources for journalists, or as contributors to blogs or websites, scientists face the challenge of being relevant and comprehensible for a lay audience. Finally, because of the practical implications of expert advice, scientists as public experts often find themselves intertwined with political and commercial interests – an experience not cherished by all scientists.