Grids are becoming the platform of choice for high performance computing. Although grids present a unified view of resources, they need evolved user interfaces in order to fully take advantage of their potential for real applications. Grid portals can deliver complex grid solutions to users; they do so without the need to download or install specialized software, or worrying about setting up networks, firewalls, and port policies. Due to the powerful, general-purpose nature of grid technology, and the nature of the grid resources they expose, the security of portals or points of access to such resources must be carefully considered. In this paper we present GiPS, the SUMA/G Grid Portal, a user-specific portal which allows Java applications to access grid resources for execution. We describe how the portal exploits standard, off-the-shelf commodity software together with existing grid infrastructures in order to facilitate security and data access. The main technologies used by GiPS are GSI, MyProxy, Java CoG Kit, GridSphere, and SUMA/G middleware. In SUMA/G, Java classes and files are loaded on demand from the user's machines. We describe how the SUMA/G Grid Portal supports this execution model and how it allows users to access controlled external data servers (users' local file systems and file systems accessible from their local workstations), under a secure platform. © Springer-Verlag Berlin Heidelberg 2007.