Ambient Networks target forthcoming dynamic communication environments where a multitude of different wireless devices, radio access technologies and network operators can cooperate as well as compete by means of instant inter-network agreements. This kind of arrangement, referred to as composition, makes great demands on the integration of radio access technologies and on the architecture of networks addressing access heterogeneity. In order to facilitate such a dynamic composition of access networks, a Multi-Radio Access (MRA) architecture has been devised consisting of Multi-Radio Resource Management (MRRM) and Generic Link Layer (GLL) functionality. One of the key objectives of the MRA architecture is the efficient utilization of the multi-radio resources by means of effective radio access selection mechanisms. In this paper the functions of MRRM and GLL and their role in multi-radio access selection are explained and some overall results from a series of evaluation studies are presented. Finally, a target architecture that supports the most promising concepts based on the results from the evaluation studies is outlined. This work has been performed as part of the Ambient Networks project, an integrated project within the EU IST 6th Framework Programme.