This chapter will explore the contested nature of science in the sustainable development process. Key illustrations will be drawn from the debates and controversies over climate change and genetic modification. The cultural and sometimes national context of these debates are also important as the discussion of sustainability in Russia demonstrates. The concept of risk, the precautionary principle and the theory of reflexive modernization will also be examined. The idea that sustainability is not a scientific concept as such, although it is, and should be, informed by it, is an underlying assumption of this chapter, even though in practice sustainability, and perhaps science too, is also a political act – but this, as with so much, is a matter for dialogue and discussion.