The United Kingdom is an international leader in the application of environmental policy appraisal (EPA). From the late 1980s until 2004, the UK's system sought to produce ex-ante assessments of the potential environmental impacts of different policy options. Critics maintain that it had a very limited impact on policy making activities in Whitehall departments. However, the empirical basis for these claims and the literature about EPA in general is surprisingly limited. This paper seeks to better understand what facilitates or retards EPA by looking at its use in Whitehall. It thus allows us to draw lessons for the UK's new and more integrated appraisal regime, as well as similar systems such as that now being rolled out by the European Commission. We find that the implementation of EPA was both weak and highly sectorised, and that there is an underlying resistance to policy appraisal per se in Whitehall. These weaknesses urgently need to be addressed otherwise the new system of integrated appraisal will not deliver all that is expected of it.