This essay offers a critical engagement with the ideal of policy relevant environmental knowledge. Using examples in environmental governance and conservation, it argues that by packaging knowledge in terms and categories that are considered politically salient, scientists do not just inform policy-making by providing information about presumed pre-existing objects in nature and environment; rather, science is constitutive of those objects and renders them amenable for policy and governance. These political implications of scientific knowledge imply a need for critical scrutiny of the interests that science serves and fails to serve as well as mechanisms to ensure the accountability of science. This essay is a modified and expanded version of the inaugural lecture with the same title that was delivered on June 2, 2016 at Wageningen University, the Netherlands.