Second generation Web portals distinguish themselves from first generation ones for their architecture, which is component-oriented. In particular, the basic component constituting them is often referred to as portlet. The portal is responsible for aggregating information coming from different sources, local or remote, available in the form of mark-up fragments. Each of those fragments is produced by a portlet. In the context of Web portals, the possibility to deploy a portlet in any portal is particularly significant. To this extent, that is, to achieve interoperability among portals, it has been necessary to define a standard way to develop and deploy portlets.