Pseudonym technology is attracting more and more attention and, together with privacy violations, is becoming a major issue in various e-services. Current e-service systems make personal data collection very easy and efficient through integration, interconnection, and data mining technologies since they use the user's real identity. Pseudonym technology with unlinkability, anonymity, and accountability can give the user the ability to control the collection, retention, and distribution of his personal information. This chapter explores the challenges, issues, and solutions associated with pseudonym technology for privacy protection in e-services. To have a better understanding of how the pseudonym technology provides privacy protection in e-services, we describe a general pseudonym system architecture, discuss its relationships with other privacy technologies, and summarize its requirements. Based on the requirements, we review, analyze, and compare a number of existing pseudonym technologies. We then give an example of a pseudonym practice - e-wallet for e-services and discuss current issues.