On January 1, 2005 the EU ETS was launched. It is a multinational emission trading system helping participating countries to reach their Kyoto targets. The first three years after its implementation was Phase I, a learning phase offering a trial-and-error approach for the following compliance period. Despite its weaknesses and the many obstacles and problems along the path from development to realization, most critics agreed that it is a success. This work gives an overview and evaluates the challenges and opportunites such a system yields.