This paper develops a simple model of software piracy to analyze the short-run effects of piracy on software usage and the long-run effects on development incentives. We consider two types of costs associated with piracy: the reproduction cost that is constant across users and the degradation cost that is proportional to consumers' valuation of the original product. We show that the effects of piracy depend crucially on the nature of piracy costs. Policy implications concerning copyright protection are also discussed. JEL Classification: O34, L1, L86.