  Long-duration gamma-ray bursts (long-GRBs) can be detected throughout cosmic
history and provide several unique insights into star-formation and galaxy
evolution back to the era of reionization. They can be used to map star
formation, identify galaxies across the luminosity function, determine detailed
abundances even for the faintest of galaxies, quantify the escape fraction of
ionizing radiation and track the progress of reionization. Fully exploiting
these techniques requires a significant increase in the number of long-GRBs
identified and characterised at $z\gtrsim6$, which can be achieved through a
discovery mission with the capabilities of THESEUS, in combination with the
powerful follow-up facilities that will be available in the 2030s.
