Context-aware computing has been a rapidly growing research area, however its uses have been predominantly targeted at pervasive applications for smart spaces such as smart homes and workplaces. This research has investigated the use of location and other context data in access control policy, with the purpose of augmenting existing IP and application-layer security to provide fine-grained access control and effective enforcement of security policy. The use of location and other context data for security purposes requires that the technologies and methods used for acquiring the context data are trusted. This thesis begins with the description of a framework for the analysis of location systems for use in security services and critical infrastructure. This analysis classifies cooperative locations systems by their modes of operation and the common primitives they are composed of. Common location systems are analyzed for inherent security flaws and limitations based on the vulnerability assessment of location system primitives and the taxonomy of known attacks. An efficient scheme for supporting trusted differential GPS corrections is proposed, such that DGPS vulnerabilities that have been identified are mitigated. The proposal augments the existing broadcast messaging protocol with a number of new messages facilitating origin authentication and integrity of broadcast corrections for marine vessels. A proposal for a trusted location system based on GSM is presented, in which a model for tamper resistant location determination using GSM signaling is designed. A protocol for association of a user to a cell phone is proposed and demonstrated in a framework for both Web and Wireless Application Protocol (WAP) applications. After introducing the security issues of existing location systems and a trusted location system proposal, the focus of the thesis changes to the use of location data in authorization and access control processes. This is considered at both the IP-layer and the application-layer. For IP-layer security, a proposal for location proximity-based network packet filtering in IEEE 802.11 Wireless LANs is presented. This proposal details an architecture that extends the Linux netfilter system to support proximity-based packet filtering, using methods of transparent location determination through the application of a pathloss model to raw signal measurements. Our investigation of application-layer security resulted in the establishment of a set of requirements for the use of contextual information in application level authorization. Existing network authentication protocols and access control mechanisms are analyzed for their ability to fulfill these requirements and their suitability in facilitating context-aware authorization. The result is the design and development of a new context-aware authorization architecture, using the proposed modifications to Role-based Access Control (RBAC). One of the distinguishing characteristics of the proposed architecture is its ability to handle authorization with context-transparency, and provide support for real-time granting and revocation of permissions. During the investigation of the context-aware authorization architecture, other security contexts in addition to host location were found to be useful in application level authorization. These included network topology between the host and application server, the security of the host and the host execution environment. Details of the prototype implementation, performance results, and context acquisition services are presented.