This paper is an economist’s guide to auctions on the Internet. It traces the development of online auctions since 1993, and presents data from a comprehensive study of 142 different Internet auction sites. The results describe the transaction volumes, the types of auction mechanisms used, the types of goods auctioned, and the business models employed at the various sites. These new electronic-commerce institutions raise interesting questions for the economic theory of auctions, such as predicting the types of goods to be sold at auction, examining the incentive effects of varying auctioneer fee structures, and identifying the optimal auction formats for online sellers.