ELeGI, the European Learning Grid Infrastructure, has the ambitious goal of fostering effective learning and knowledge construction through the dynamic provision of service-based contextualised and personalised learning environments. The success of this venture will depend to a considerable extent on the usability of such environments, and their usability in turn will depend on a successful strategy for the dynamic integration and maintenance of sets of services. The concept of the portal is therefore of considerable interest, as it is often portrayed as a means whereby a user can access an integrated set of related information and services. This paper reviews the portal concept with a view to its suitability as a design basis for enabling technology that will address usability concerns. The paper proceeds by summarising the usability requirements of learning environments, reviewing some of the ideas currently associated with different types of portals (enterprise portals, institutional portals, user-centric portals, Grid portals), and concludes by deriving a taxonomy of portal characteristics against which the usability requirements of ELeGI can be assessed.