This paper has benefited from helpful comments made by anonymous referees and by the participants of European Policy on Intellectual Property Conference (Munich, 2003), Association Française de Science Economique Congress (2003), Conference in Honour of Zvi Griliches (Paris, 2003).Economic theory views patents as policy instruments aimed at fostering innovation and diffusion. Three major implications are drawn regarding current policy debates. First, patents may not be the most effective means of protection for inventors to recover R&D investments when imitation is costly and first mover advantages are important. Second, patentability requirements, such as novelty or non-obviousness, should be sufficiently stringent to avoid the grant of pa...