The technological promise constitutes a specific form of scientific communication and mediation of knowledge. Used extensively to promote certain emerging sciences and technologies such as nanotechnologies and synthetic biology, it predicts profound long-term changes. This form of communication impacts, in turn, debate on knowledge and on protest concerning science. It transforms discourses, argumentative practices, and modes of involvement or organization of critics with regard to emerging technologies. A major aim of this critique is therefore to challenge the conceptions of timescales and technological progress underlying promissory communication.