How do we react to uncomfortable futures? By developing the notion of chronopolitics, this article presents two ways that we typically react to future challenges in the present. At the core of the chronopolitics of prevention, we find a striving for normalization and conservation of the present vis-à-vis dangerous futures. In contrast, the chronopolitics of preemption are geared towards a reformation, if not even a revolution of the present. Two case studies in the field of science and technology policy illustrate (...) the difference between prevention and preemption. The debate on human embryonic stem cells illuminates prevention. The debate on nanotechnology clarifies preemption. (shrink)