The quality of evidence in conservation decision making’. Conservation science is a crisis discipline in which scientists measure impacts in terms of species extinctions and ecosystem collapse, and practitioners typically seek solutions that preempt irreversible change. Present challenges such as global losses of biodiversity and social-ecological systems require efficient and timely action. Decisions need to be made quickly, yet the data and understanding necessary to assess problems and provide unequivocal solutions are typically unavailable, incomplete, dated, or biased. These issues are amplified by a suite surprisingly common procedures and actions that – without malicious intent – misuse or misrepresent data and analyses, generating spurious results and misleading advice. Here, I review the quality and reliability of expert opinion used to fill information gaps and assess the quality of scientific evidence, present some recent empirical results on the limits of ecological judgement, and discuss the prospects for improving expert judgement.