Grand corruption poses a serious threat to international efforts to tackle climate change. This Helpdesk Answer provides an overview of evidence on grand corruption in climate change policies, focusing on three broad themes: i) energy transition, including decarbonisation, renewable energy and critical minerals; ii) biodiversity loss; and iii) climate finance. This answer structures each section based on the forms of grand corruption, looking at state capture, regulatory capture and institutionalised grand corruption, as well as unequal access to lobbying and the transnational component of grand corruption and illicit financial flows related to grand corruption. The answer presents evidence of grand corruption in all three themes, with differences in specific manifestations. For example, there is evidence of state capture in decarbonisation policies with regards to carbon tax adoption; of institutionalised grand corruption in renewable energy transition; and transnational corruption through foreign bribery in relation to biodiversity loss, to name a few. There are broad negative consequences of grand corruption in climate change policies, including environmental degradation, health risks, human rights violations, suboptimal allocation of resources and delays in green energy transition.