Web-based Information Systems (WIS) are used for diffusing and processing information over the Internet. Because of the large amounts of information they manage, it is crucial to adapt the delivered information to users and to give them a progressive access to information. For this purpose, we propose to stratify the Information Space of a WIS by decomposing it into personalized sub- Information Spaces. Multimedia data can also be stratified into multiple formats used for their presentation. These stratifications are described through a Progressive Access Model (PAM) written in UML. In order to personalize the progressive access, the PAM is linked to both the data model of the application domain and to a Generic User Model describing the rights and preferences of users. We also show that, based on these stratifications, navigation mechanisms allow users to access first some minimum and essential information, and then larger and/or smaller personalized Information Spaces.