This monograph reviews the existing body of knowledge with regard to theory and practice of online auctions. Auctions are an excellent mechanism to explore and determine the value of goods or services when there is a high uncertainty about the real value of these goods or services. Simply put, it is assumed that the price of a product or service reflects the value and the winner is the one who offers the highest price - the reality in a business context is much more complex. The value has multiple dimensions of which price is only one factor. The highest bidder is not always the winner. The winner is sometimes the loser. The value of a product or service depends on the total value created in a business network. In a business network several actors will create value for the end-customers and online auctions may create value for all actors involved. Advanced information and communication technologies may provide a reduction of transaction costs for firms and may provide opportunities to create new business models in which online auctions play a crucial role.The objective of Theory and Practice of Online Auctions is to provide an overview of state-of- the-art research with regard to online auctions. First, the author discusses the move to onlineauctions. In most industries - partly under the influence of the Internet - the move was madefrom mainly bilateral negotiations to a mixed use of online bilateral negotiations and online auctions. Section 2 provides the state-of-the-art of scientific research. Three perspectives are discussed: (i) online auctions as standalone mechanisms, (ii) online auctions in a broader context, and (iii) online auctions in the context of the firm and its business network. In particular the role of information and communication technologies is analyzed. Section 3 presents the latest modeling studies of online auctions. Three types of online auctions are analyzed: online forward auctions, online reverse auctions, and online double auctions. In section 4 design aspects of online auction markets are discussed. In section 5 a number of key applications used both by businesses and consumers of online auctions are presented. The last section presents a short summary and directions for future research.