  Gamma-ray Bursts (GRBs) are the most powerful transients in the Universe,
over-shining for a few seconds all other $\gamma$-ray sky sources. Their
emission is produced within narrowly collimated relativistic jets launched
after the core-collapse of massive stars or the merger of compact binaries.
THESEUS will open a new window for the use of GRBs as cosmological tools by
securing a statistically significant sample of high-$z$ GRBs, as well as by
providing a large number of GRBs at low-intermediate redshifts extending the
current samples to low luminosities. The wide energy band and unprecedented
sensitivity of the Soft X-ray Imager (SXI) and X-Gamma rays Imaging
Spectrometer (XGIS) instruments provide us a new route to unveil the nature of
the prompt emission. For the first time, a full characterisation of the prompt
emission spectrum from 0.3 keV to 10 MeV with unprecedented large count
statistics will be possible revealing the signatures of synchrotron emission.
SXI spectra, extending down to 0.3 keV, will constrain the local metal
absorption and, for the brightest events, the progenitors' ejecta composition.
Investigation of the nature of the internal energy dissipation mechanisms will
be obtained through the systematic study with XGIS of the sub-second
variability unexplored so far over such a wide energy range. THESEUS will
follow the spectral evolution of the prompt emission down to the soft X-ray
band during the early steep decay and through the plateau phase with the unique
ability of extending above 10 keV the spectral study of these early afterglow
emission phases.
