The EU emissions trading system (ETS) is based on an EU Directive that was adopted in 2003 and started functioning in 2005. It caps industrial emissions and allows trade of emission rights. EU officials refer to the ETS as both the ‘cornerstone’ and the ‘flagship’ of EU climate policy. The ETS has now been functioning for over five years and overall results stand out as mixed. The role of different societal actors can shed light on this: much industry was reluctant; environmental organisations critical; member states cautious; EU bodies wing-clipped; and matching US efforts lacking. Although a substantially improved design has been adopted for the post-2012 phase and prospects ahead look promising, several complex interaction effects mean a need for continued great but also cautious and sober expectations.