Web services technologies are advancing fast and being extensively deployed in many di?erent application environments. Web services based on the eXt- sible Markup Language (XML), the Simple Object Access Protocol (SOAP), andrelatedstandards,anddeployedinService-OrientedArchitectures(SOAs) are the key to Web-based interoperability for applications within and across organizations. Furthermore, they are making it possible to deploy appli- tions that can be directly used by people, and thus making the Web a rich and powerful social interaction medium. The term Web 2.0 has been coined to embrace all those new collaborative applications and to indicate a new, “social” approach to generating and distributing Web content, characterized by open communication, decentralization of authority, and freedom to share and reuse. For Web services technologies to hold their promise, it is crucial that - curity of services and their interactions with users be assured. Con?dentiality, integrity,availability,anddigitalidentitymanagementareallrequired.People need to be assured that their interactions with services over the Web are kept con?dential and the privacy of their personal information is preserved. People need to be sure that information they use for looking up and selecting s- vicesiscorrectanditsintegrityisassured.Peoplewantservicestobeavailable when needed. They also require interactions to be convenient and person- ized, in addition to being private. Addressing these requirements, especially when dealing with open distributed applications, is a formidable challenge.