[en] Designed to be compatible with competition mechanisms, tradable green certificates markets appear a priori the appropriate incentive scheme to promote renewable electricity in the liberalized electric markets. Although they present many theoretical advantages on other incentive instruments for the support of renewable energy, their implementation and design raise some difficulties. Their actual effectiveness still has to be confirmed by the on-going experiences in several member states, notably regarding to their ability to create an incentive and stable framework to investors. (author)