The European Theoretical Spectroscopy Facility addresses an important need of European science and technology by providing experimental, industrial and other researchers with access to state-of-the-art computer simulation tools for electronic excited states in matter, together with high-quality support from ETSF personnel, mirroring the massive progress in the power and resolution of new European experimental facilities. All domains that need knowledge about electronic excitations, transport and spectroscopy will benefit from the ETSF, such as condensed matter physics and chemistry, biology, materials science and nanoscience, atmospheric science, and astrophysics. The ETSF provides users with computer codes, background knowledge, customised support and development, training, and collaborators to enhance their studies of the electronic and transport properties of complex or nanoscale materials. Its focus is on the rapid transfer of ground-breaking fundamental knowledge of matter, at the quantum-mechanical level, to detailed understanding and future-oriented design of prototypical or technologically relevant systems. The ETSF has been successfully designed and recently brought into operation by the Nanoquanta Network of Excellence with the support of national and local institutions. In the present ETSF-I3 project, the ETSF is partnered by the Barcelona Supercomputing Centre to create a framework for deploying the ETSF infrastructure to a much wider range of users, through user training and projects supported by ETSF scientists. The ETSF-I3 project will monitor the scientific and technological needs of users, and will boost the user-oriented development of ETSF software, algorithms and libraries made available on the most advanced computational platforms. ETSF-I3 will be crucial to keep the ETSF at the forefront of knowledge and establish it as the world-wide reference centre for modelling of electronic excited states.