Grid computing can help to promote high-performance computing at a low overall cost by encouraging research centers to share their resources. However, research staff usually finds it quite hard to use Grids effectively, due to the need of installing and managing new Grid software. Thus, Grid portals are created, making it easier to take advantage of the full capability of the Grid, favoring in this way its use. The goal of this paper is to describe the process of design and implementation of a Grid Portal with the aim of both supporting distributed high-performance resources and make its use by researchers as transparent as possible. This portal uses standard Grid and Web technologies. We have designed the portal so that it can be adapted to different existing Grid infrastructures, based on the Globus Toolkit, and new functionalities can be easily added. The first prototype of the portal has been tested on an experimental Grid platform, and we present encouraging experiences carried out there.