This chapter considers the requirements for security in business-to-consumer e-commerce systems. Experience to date has revealed that these services are potentially vulnerable to a wide range of Internet-based threats. Some of these, such as data streaming and phishing, can be closely tied to the e-commerce domain, whereas others (such as malware, denial-of-service attacks, and Web site defacement) represent more generally applicable problems that nonetheless assume additional significance in an e-commerce context. The existence of the threats demands a variety of safeguards. Technical measures can be used to ensure the confidentiality and integrity of transactions and to support authentication of the parties involved. In addition, supporting measures are considered valuable to raise consumer awareness of security and boost confidence in e-commerce services. It is concluded that e-commerce represents an area of continued growth, and potential threats are likely to grow rather than diminish. As such, appropriate attention by service providers is crucially important.