The EGI Design Study represents a project for the conceptual setup and operation of a new organizational model of a sustainable pan-European grid infrastructure. The project will address key questions concerning the assessment of technical and financial feasibility of such a sustainable grid service, producing a "conceptual design report" addressed at member state policy makers and funding agencies. The project will interact and encourage the member states to make the strategic decisions required to establish and support a sustainable grid infrastructure, and initiate its implementation.Europe has developed a scientific grid infrastructure in and across many member states which is being used by many research communities. This has come about by investments from member states into national resources as well as by the European Commission with project awards for European flagship projects such as EGEE (Enabling Grids for E-sciencE) and support projects to develop interoperable middleware technologies and co-ordination mechanisms. At present and on a project-basis, this production service infrastructure cannot be sustained indefinitely, and there is a clear need to establish a long term technical and financial structure. This need is confirmed by a series of recommendations from the e-Infrastructure Reflection Group (e-IRG), and the European Grid Initiative (EGI) represents the coordinated effort to advance on this matter.The EGI Design Study will identify the conditions and develop the framework for a sustainable Grid infrastructure which should take over the coordination and operation of the future European grid infrastructure from today's projects. The design study will foster the emergence of a new organisational model designed to consolidate a sustainable approach to e-Infrastructures, in particular in the domain of grids, facilitating new service provisioning schemes, more application neutral and open to all user communities and resource providers.