The ETSF is a knowledge center for theoretical spectroscopy and a network of researchers carrying out state-of-the-art research on theoretical and computational methods for studying electronic and optical properties of materials. The ETSF gathers the experience and know-how of more than 200 researchers in Europe and the United States, facilitating collaborations and rapid knowledge transfer. Highly efficient computational software plays a crucial role in bridging the gap between theoretical methods and real applications.