The initial years of the European Union's Emissions Trading System (EU ETS) have provided a large-scale testing ground for trading of a new environmental commodity, carbon dioxide. This paper provides an overview of the origins and characteristics of the EU ETS. It then goes on to analyse the most contentious issues that have been discussed in the economics literature and in the public debate surrounding the trading system. The lessons learned are diverse and not all experiences are positive. Nevertheless, invaluable information has been gained from the EU ETS and policy makers in Europe and elsewhere would be wise to make use of it, be they supporters of emissions trading or sceptics to such policies. The paper concludes with a look toward the future, highlighting some upcoming revisions of the EU ETS and at what issues remain unresolved.