For over the past twenty years, the United States has witnessed a pro-patent movement. In response, numerous concerns have been raised, including possible impediments to innovation in cumulative technologies, emergence of anti-commons, barriers to entry and an elevation of costs of innovation associated with defensive patenting, growth in patent litigation and poor quality patents. Although there is little systematic evidence that these concerns have materialized in any substantial way, vigilance is nonetheless warranted. © Springer Science+Business Media, Inc. 2004.