Web-based services are vulnerable to a number of attacks. While providers of these services employ countermeasures (such as firewalls, encryption, and authentication systems) to reduce security risks, some of these security measures can be rendered useless if the PC of a user that accesses such a web-based service is not properly secured. Malicious software that is installed on a user’s PC, for example, can potentially circumvent existing protection measures by recording login credentials and impersonating the victim. To counter threats that are arising through client PCs, many providers of security sensitive web-based services have introduced usage policies for their services. These policies require users to ensure that their PCs are in a proper security state (e.g. the PC is equipped with an up-to-date anti-virus application, a personal firewall, and all security updates have been installed). However, service providers have no possible means of enforcing these policies and they have to rely on users to check the security state of their PCs manually. This thesis presents a mechanism that allows a service provider to remotely measure the security state of a user’s PC. This mechanism is based on Trusted Network Connect (TNC). TNC is a network access control mechanism that takes the security state of an access requesting party into account before making an access decision. However, TNC is currently limited to closed environments such as LANs and VPNs. This thesis proposes solutions based on authentication standards for enabling TNC in open, web-based scenarios. In particular, an architectural model for TNC is proposed that takes additional security and privacy requirements into account. Furthermore, a communication scheme is proposed that is based on standardised protocols and message formats. These protocols and message formats have been leveraged to allow web-based TNC checks to be triggered through aWeb browser and TNC messages to be exchanged. These building blocks have been combined into a prototype implementation which has been evaluated using a test bed approach. This prototype successfully demonstrated that TNC can be adapted to web-based environments where it provides assurance as to the security state of clients accessing security sensitive web-based services.