In this dissertation, I explore bidders’ behavior in multiple auctions which are conducted sequentially or simultaneously. The first and the second chapters examine buyers’ bidding behaviors in an environment of multiple simultaneous auctions and show that the wildly-used assumption of proxy bidding is inappropriate in the multiple auction setting. The first chapter proposes two models which try to describe online auction platforms. One model has a fixed ending time and the other does not. I show that incremental bidding strategy can arise out of equilibrium and weakly dominate the proxy bidding strategy. Late bidding is also discussed. I use the data I collect from eBay to test these theoretical predictions in the second chapter. The estimation results basically support the theory part. Incremental bidders who switch among different auctions are more likely to win and have higher payoffs than proxy bidders. The third essay studies the procurement auctions in the Texas school milk market. I define score functions to map the bids from multiple dimensions to one dimension and analyze the factors that may affect the bids of school milk suppliers. After considering the impacts of these factors including backlogs and cost synergies, I can still find some evidences for existence of collusion among the bidders.