This chapter explores the contribution diplomacy has made to the EU’s external environmental and climate policies. By studying the EU’s activities in the area of environmental diplomacy, we can elaborate upon the factors that influence the effectiveness or otherwise of the EU’s external environmental policies. The chapter traces the development of the EU’s key institutional mechanism for coordinating environmental diplomacy, the Green Diplomacy Network. It also uses the case of the EU’s involvement in the COP21 climate conference to illustrate how diplomacy matters for effectiveness of EU external environmental relations. Climate change is a special case of environmental diplomacy because it has attracted very high global attention compared with other environmental issues.