The implementation of multiple Grid computing portals has led us to develop a methodology for Grid portal development that facilitates rapid prototyping and building of portals. Based on the NPACI Grid Portal Toolkit (GridPort) and the NPACI HotPage, all portals inherit interactive Grid services, share a single account and login environment, and share the infrastructure required to support and provide services used by each portal. We have demonstrated that the GridPort software can be used in production application portal environments, that the software can be configured to extend multiple sites. In this paper, we describe our experiences gained in building Grid portals and developing software for the Grid. We describe the architecture and design of the portal system, Grid services and systems employed, as well as the unique features of the system. We present descriptions of several application portals and the driving design choices. Finally, we discuss the new and emerging architectu...