Interstate war data are critical for many important research areas in international relations. Scholars working in these areas frequently rely on the Correlates of War (COW) interstate war data. This article analyzes the COW interstate war data version 4.0, covering the years 1816 through 2007. Using COW coding criteria, the article finds that in more than 30 percent of the ninety-five COW interstate wars, codings of at least one of the key variables—existence of the war, list of participants, initiator, and outcome—needs to be revised. This article describes a revised data set on interstate wars that incorporates these revisions. The data set includes a metric for breaking apart large, multilateral wars such as the World Wars into component wars. For each participant in multilateral wars, it provides information on exactly which states that participant fought, and specific initiation and outcome codings for that participant.