Abstract: International airline alliances allow airlines to coordinate their operations in providing international service. This paper analyzes the effect of such alliances on traffic levels, fares, and welfare. In the model, the benefits of alliances arise because cooperative pricing of trips by the partners puts downward pressure on fares in the interline city-pair markets (these are markets where travel on both carriers is necessary). The loss of competition in the interhub market, which connects the hub cities of the partners, generates a countervailing effect, tending to raise the fare in that market. While the presence of economies of traffic density complicates these impacts by generating cost links across markets, simulation analysis shows that the above tendencies typically prevail. Welfare analysis shows that both consumer and total surplus typically rise following formation of an alliance despite the harm to interhub passengers, suggesting that the positive effects of alliances may outweigh any negative impacts.