Location-based mobile services let wireless mobile users access Web-based information about resources in their immediate vicinities. The authors describe an algorithm that draws on context mobility elements, such as the user's travel direction and speed, to form personal service areas. Their experimental context-aware tourist information system (CATIS) leverages XML technologies and Web services to provide tourist information to mobile users based on these personal service areas and the users' preferences. Because Web service performance depends on the underlying databases, the authors also developed a layered caching scheme for storing environmental data to improve response time.