**5. Conclusions and Perspectives**

THESEUS mission concept aims to fully exploit the transformative potential of GRBs for investigating the Early Universe and multi-messenger astrophysics, while simultaneously vastly expanding the discovery space of most high energy transient phenomena over the entirety of cosmic history and allowing tests of fundamental physics. THESEUS will achieve these ambitious goals through a step change in capabilities for the detection and characterisation of GRBs and other transients over a very broad energy band (0.3 keV to 10 MeV) and wide field of view, including on-board near-infrared imaging and spectroscopy, and is designed to be at the forefront of these science fields in the late 2030s.

THESEUS will also enable huge synergies with the premier future observatories (e.g., ELT, ATHENA, CTA, the next generation GW and neutrino telescopes), providing simultaneous wide sky monitoring, rapid follow-up and real-time alerts. A broad range of other science programmes will be enabled by THESEUS, including using observations of GRB emission as laboratories of ultra-relativistic matter and, e.g., for testing Lorentz invariance, as well as gathering statistics on large populations of other high energy sources and transients. Thus, THESEUS data will be of interest to a very wide user community, also through its open guest-observer programme.

The European leadership in these scientific and technological area builds on past success and ongoing investment: in addition to the involvement in pioneering satellite missions (BeppoSAX, HETE-2, XMM-Newton, INTEGRAL, Swift, SVOM), Europe has been at the heart of developments in multi-messenger astrophysics (e.g., Virgo, Einstein Telescope, KM3Net) and in electromagnetic follow-up via European-led consortia (e.g., ENGRAVE, VINROUGE, STARGATE). THESEUS is one of the three mission concepts that were selected by ESA in 2018 for a Phase A study as candidates for the M5 mission. The ESA Phase A study as candidate M5 mission, conducted from Fall 2018 to Spring 2021 led to detailed, workable and well qualified solutions for the spacecraft, its payload and operations. It has also demonstrated the technical and programmatic feasibility of accomplishing the core science goals with this mission concept. Based on these great achievements and heritage, the THESEUS project is being further developed for responding to new opportunities for medium-class missions. The later launch schedule with respect to M5 of these new opportunities will further improve the scientific return of THESEUS for multi-messenger astrophysics and time-domain astronomy, allowing for a great synergy with third generation GW detectors (expected to begin operations only in the second half of the 2030s) and improving the synergy with, e.g., the ATHENA and LISA space observatories (expected to be launched in the mid '30s), while maintaining the relevance of the other key science goals.

**Author Contributions:** Conceptualization, L.A.; writing–original draft preparation, G.S.; formal analysis, G.S., M.B., R.C.; supervision, N.T., E.B., D.G., P.O. and A.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by ASI and INAF through agreement n 2018-29-HH.0., by the UE H2020 programme through the AHEAD2020 project and by the national space agencies and institutions supporting the THESEUS Consortium.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors, on behalf of the entire THESEUS consortium, are grateful to the ESA appointed THESEUS Science Study Team, as well as the entire ESA Study Team, for all their work and support across the mission assessment phase (2018–2021). The authors acknowledge the anonymous referees for their useful comments.

**Conflicts of Interest:** The authors declare no conflict of interest.
