With the exponential increase of information on the web, the emphasis has shifted from mere viewing of information to efficient retrieval and notification of selective information. Currently, users have to poll the pages manually to check for changes of interest, resulting in waste of resources and associated high cost. Hence, an efficient and effective change detection and notification mechanism is needed. WebVigiL, a general-purpose, active capability-based information monitoring and notification system, handles specification, management, and propagation of customized changes as requested by a user. The emphasis of change detection in WebVigiL is to detect customized changes on the document, based on user intent. In this paper, we propose two different algorithms to handle change detection to contents of semi-structured and unstructured documents. Though the approach taken is general, we will explain the change detection in the context of HTML (unstructured) and XML (semistructured) documents. We also provide a simple change presentation scheme to display the changes computed. We highlight the change detection in the context of WebVigiL and briefly describe the rest of the system.