  The proposed THESEUS mission will vastly expand the capabilities to monitor
the high-energy sky, and will exploit large samples of gamma-ray bursts to
probe the early Universe back to the first generation of stars, and to advance
multi-messenger astrophysics by detecting and localizing the counterparts of
gravitational waves and cosmic neutrino sources. The combination and
coordination of these activities with multi-wavelength, multi-messenger
facilities expected to be operating in the thirties will open new avenues of
exploration in many areas of astrophysics, cosmology and fundamental physics,
thus adding considerable strength to the overall scientific impact of THESEUS
and these facilities. We discuss here a number of these powerful synergies.
