Persistent gaps in the evidence base regarding the performance of conservation policies has put pressure on the conservation policy field to adopt 'best practice' programme evaluation methods. These are methods that account for the counterfactual and are able to attribute causality between a conservation policy and specific observable environmental and social impacts. Despite this pressure, use of such methods continues to be rare. This paper uses the Delphi technique to provide the first systematic assessment of the reasons behind the apparent hesitation of conservation practitioners to adopt rigorous policy impact evaluation methods. The Delphi study consisted of two online questionnaires conducted on conservation policy experts. The results presented confirm that the use of rigorous impact evaluation methods in conservation is still very limited but this, crucially, is not because conservationists are ignorant of these methods or their advantages. In fact, considerable effort is being made to develop and improve evidence standards but these efforts have largely been thwarted by large financial and time related constraints that mean even elementary evaluations are hard to achieve. The results from this Delphi study allow us to provide more realistic recommendations on how impact evaluation studies can be more widely embraced and implemented in conservation practice.