The Patent Holder wishing to enforce her patent has several ways of doing so. In the world of patent litigation, however, one of the most important remedies is the preliminary injunction (PI), whereby an allegedly infringing competitor is forced to stop selling the goods in the market in the interim period before the court reaches its final decision on the merits. In spite of this, the economic literature has afforded little attention to PIs. This article uses a simple economic model to investigate how a Patent Holder and an Alleged Infringer will behave with and without the PI instrument. We show that party behavior depends on the probability that the Patent Holder does indeed have a valid patent and will prevail in a final court decision and on the extent to which courts can determine damages correctly. We find that while patent rights benefit the Patent Holder, the PI instrument to a large extent benefits the Alleged Infringer. It does so by insuring him against large damages payments and allowing him to receive compensation for actions not taken, i.e. for not being on the market in the interim period before the final court decision. Finally, we discuss different decision rules a court could use to decide whether to grant a PI, and propose a decision rule whereby courts can take into account the social benefits or losses of an erroneous PI decision.