Every time you bid on eBay, watch a house go under the hammer, or hear about a telecom company winning spectrum rights, you are witnessing an auction. But behind the gavel and the bids lies a deep strategic puzzle: how much should you offer when you do not know what others are willing to pay, and how should a seller set the rules to get the best outcome? This chapter explains the game theory of auctions—why different formats lead to very different bidding behaviour, and when they all end up giving the seller the same expected profit anyway.