Reputation Management is rather a new field in the realm of computer security. This is mainly due to the fact that computers and software are more and more involved in open networks. In these open networks, any encountered user may become part of the user base and associated interactions even if there is no a priori information about those new users. The greater openness of these networks implies greater uncertainty regarding the outcomes of these interactions. Traditional security mechanisms, for example, access control lists, working under the assumption that there is a priori information about who is allowed to do what, fail in these open situations. Reputation management has been used for ages in the real world to mitigate the risk of negative interaction outcomes among an open number of potentially highly profitable interaction opportunities. Reputation management is now being applied to the open computing world of online interactions and transactions. From the first edition of this book chapter written in 2008 to this second one written in 2012, online reputation services have gained a lot of momentum and reputation monitoring and analysis is now intensively sold to brands companies by communication and marketing agencies. After an introductory section, the second section of this chapter discusses the general understanding of the notion of reputation. Section 3 explains where this concept of reputation fits into computer security. The fourth section presents the state of the art of robust computational reputation. Section 5 gives an overview of the current market of online reputation services, also now called e-reputation services. The conclusion underlines the need to standardize online reputation for increased adoption and robustness.