Abstract: Methods and apparatus for monitoring the on-line activities of an on-line user are disclosed. Information regarding the user's activities are used as a basis for targeting advertisements to the user. Monitoring is performed by a client monitoring application, which performs monitoring without intruding on the user's activities. In monitoring, URLs in the address bar of the browser application are obtained by the client monitoring application, or the stream of URLs transmitted by the browser application are obtained by the client monitoring application, or information in displayed web pages is obtained by the client monitoring application.