  THESEUS, one of the two space mission concepts being studied by ESA as
candidates for next M5 mission within its Comsic Vision programme, aims at
fully exploiting Gamma-Ray Bursts (GRB) to solve key questions about the early
Universe, as well as becoming a cornerstone of multi-messenger and time-domain
astrophysics. By investigating the first billion years of the Universe through
high-redshift GRBs, THESEUS will shed light on the main open issues in modern
cosmology, such as the population of primordial low mass and luminosity
galaxies, sources and evolution of cosmic re-ionization, SFR and metallicity
evolution up to the "cosmic dawn" and across Pop-III stars. At the same time,
the mission will provide a substantial advancement of multi-messenger and
time-domain astrophysics by enabling the identification, accurate localisation
and study of electromagnetic counterparts to sources of gravitational waves and
neutrinos, which will be routinely detected in the late '20s and early '30s by
the second and third generation Gravitational Wave (GW) interferometers and
future neutrino detectors, as well as of all kinds of GRBs and most classes of
other X/gamma-ray transient sources. In all these cases, THESEUS will provide
great synergies with future large observing facilities in the multi-messenger
domain. A Guest Observer programme, comprising Target of Opportunity (ToO)
observations, will expand the science return of the mission, to include, e.g.,
solar system minor bodies, exoplanets, and AGN.
