The proposed Design Study is a coordinated, joint effort of 16 European institutions to prepare the construction of the next-generation free-electron laser (FEL) light sources proposed as new infrastructures in France, Germany, Italy, Sweden and the UK. Th e most critical research and development for their realisation has been prioritised and grouped into six tasks dealing with photocathode electron guns and injectors, beam dynamics, synchronisation, seeding and harmonic generation, high duty-cycle supercond ucting linacs, and cryomodules technology transfer. The first goal is to develop proven designs for critical components such as the electron gun and the complete injection system, the optical system providing an optimum laser beam profile on the photocat hode, and the electron bunch compression. The second goal is the improvement of the electron beam parameters to reach the hard X-ray regime at sub-nanometer wavelengths. This also benefits FELs for longer wavelengths because it reduces the required undulat or lengths and the electron beam energy. The third objective is the development of seeding and harmonic generation techniques in order to improve the output characteristics of the new FEL sources for the benefit of the user experiments. Prerequisite to las er seeding, as well as to high resolution time-resolved experiments, is the synchronisation of all subsystems to 100 fs or better. The fourth objective is a flexible FEL pulse distribution in time in order to facilitate user experiments and enhance user ac cess. This requires the qualification of the superconducting accelerator for high duty-cycle (CW) operation. It is expected that CW operation also facilitates the precise synchronisation of the electron beam. Finally, it is necessary to move from prototy ping to industrial production of major components, in particular of complete superconducting accelerator modules. This is essential for the reliable and cost effective construction of the new facilities.