  The Cherenkov Telescope Array (CTA) is designed to be the next major
observatory operating in the Very High Energy (VHE, $\gtrsim 100$ GeV)
gamma-ray band. It will build on the imaging atmospheric Cherenkov technique
but will go much further in terms of performance than current instruments. Its
sensitivity at short timescales and the rapid repointing system will make CTA a
perfect facility to observe the gamma-ray transient sky. In this respect, the
synergies between CTA and other multi-wavelength and multi-messenger facilities
are expected to further enhance CTA's scientific scope. Thanks to its
characteristics, THESEUS will perform an unprecedented monitoring of the X-ray
variable sky, detecting, localising, and identifying transients. It will
provide external triggers and accurate location for the CTA follow-up of
transients, and their broadband characterisation, playing a key role for CTA
after the next decade.

High Energy Astrophysical Phenomena;Instrumentation and Methods for Astrophysics