We believe that navigation in information spaces is best supported by tapping into our natural spatial and geographic ways of thinking. To this end, we are developing a new computer interface model called Pad. The ongoing Pad project uses a spatial metaphor for computer interface design. It provides an intuitive base for the support of such applications as electronic marketplaces, information services, and on-line collaboration. Pad is an infinite two dimensional information plane that is shared among users, much as a network file system is shared. Objects are organized geographically; every object occupies a well defined region on the Pad surface. For navigation, Pad uses "portals" magnifying glasses that can peer into and roam over different parts of this single infinite shared desktop; links to specific items are established and broken continually as the portal's view changes. Portals can recursively look onto other portals. This paradigm enables the sort of peripheral activity generally found in real physical working environments. The apparent size of an object to any user determines the amount of detail it presents. Different users can share and view multiple applications while assigning each a desired degree of interaction. Documents can be visually nested and zoomed as they move back and forth between primary and secondary working attention. Things can be peripherally accessible. In this paper we describe the Pad interface. We discuss how to efficiently implement its graphical aspects, and we illustrate some of our initial applications.