MOSAIC: the high multiplex and multi-IFU spectrograph for the ELT

MOSAIC is the planned multi-object spectrograph for the 39m Extremely Large Telescope (ELT). Conceived as a multi-purpose instrument, it offers both high multiplex and multi-IFU capabilities at a range of intermediate to high spectral resolving powers in the visible and the near-infrared. In this talk I will showcase the key science cases that drive the top-level requirements and the adopted instrument architecture. With a planned first light in the early 2030s, MOSAIC will enable unique spectroscopic surveys of the faintest sources, from the oldest stars in the Galaxy and beyond, to the first populations of galaxies that completed the reionisation of the Universe--while simultaneously opening up a wide discovery space and providing enormous synergies with upcoming multi-wavelength facilities (e.g., Euclid, Rubin, Roman, SKA).