Governments are becoming increasingly dependent on externally produced knowledge. Their demand for knowledge has been amplified by a relatively non-ideological political climate; by the influence of international organisations sharing practice and assessing policy effectiveness; and by public opinion. Supply has grown in tandem within academia, government bodies themselves and international organisations, all hugely helped by the Web. This knowledge takes many different forms, ranging from the classic knowledge of pilots to the tacit knowledge of practitioners. How it is handled varies according to the nature of the field – some are relatively stable, closer to the natural sciences; some are fields in flux, where there is argument over even basic concepts; a third set are inherently novel, particularly those involving technology. In all areas there are unavoidable limits to the relevance and usability of knowledge, including democracy; time; the social role of ambiguity; and reflexivity. However, the shift to greater dependence on knowledge is unlikely to reverse so long as all other areas of economic and social life are changing in tandem towards greater dependence on knowledge and greater awareness of the complexities involved in any production or use of knowledge.