Today, the European Southern Observatory ESO has signed an agreement with a large international consortium for the design and construction of the Multi-Object Spectrograph (MOSAIC), an instrument for the Extremely Large Telescope (ELT). On what will be the world's largest optical telescope, MOSAIC will simultaneaously measure the light from hundreds of astronomical sources and by that trace the growth of galaxies and the distribution of matter from the Big Bang to the present day. As a consortium member, the Leibniz Institute for Astrophysics Potsdam (AIP) is responsible for part of the technical development of MOSAIC, as well as for preparing its scientific use.