The limitation of global warming below 2°C requires rapid and significant deployment of renewable energies in the power sector. Policies to support innovation and diffusion of renewables have been implemented for more than 20 years. There is currently a debate surrounding their economic and environmental efficiency and the right balance between support for innovation and support for diffusion. This article sheds light on the stakes of this debate by presenting the results of the econometric literature which evaluates the effect of these policies and compares these results with the main conclusions of non-econometric studies. The results show that innovation policies and diffusion policies have a positive impact on renewable energies and so confirm non-econometric studies. However, they reveal differentiated effects depending on the type of policies (price-based policies versus quantity-based policies) and the level of maturity of the technologies.JEL Codes: O30, O31, Q42, Q48, Q55, Q56