The intellectual property (IP) regime for software has undergone significant changes in the United States during the 1980s and 1990s, with an increase in the legitimacy of patents and a concomitant restriction of copyright in some circumstances. This study examines litigation data on patents, copyrights, and trademarks, the three main forms of US federally protected IP rights, to understand the impact of these changes and the role of intellectual property protection in software. We find a surprisingly large and still growing reliance on copyright enforcement, and evidence that incumbent firms in the industry are relatively on the defensive in patent litigation. Moreover, when compared with other similar industries, defensive strategies in software patent litigation appear to be tilted towards declaratory judgment actions, rather than mutual hold-up through counter suing. This paper also studies the interrelationship between a firm’s efforts to enforce different types of intellectual property rights. Prior research on patents and secrecy, as well as the policy debate in software, has tended to view different ...