Current Network User Interfaces (NUIs) provide entrances to an enormous amount of Web-based services, bringing about new use problems such as laborious and unsuccessful navigation. Such problems are generally more severe for users with regression of cognitive functions (eg for some elderly). This paper identifies four fundamental cognitive determinants of navigation performance that may explain these problems: situation awareness, spatial ability, task-set switching and user control of support. Based on an analysis of these demands and current support functions for navigation, three “refined” support concepts were developed: categorising landmarks, history map and navigation assistant. Via the specification of humancomputer co-operative processes and scenarios, the concepts were implemented for two rather different web-based services. The present paper provides an example implementation of the navigation assistant. The results of the study will feed into a cognitive engineering method for the design of NUIs.