The Time-Domain Extragalactic Survey (TiDES) is focused on the spectro-scopic follow-up of extragalactic optical transients and variable sources selected from forthcoming large sky surveys such as that from the Large Synoptic Survey Telescope (LSST). TiDES contains three sub-surveys: (i) spectroscopic observations of supernova- like transients; (ii) comprehensive follow-up of transient host galaxies to obtain redshift measurements for cosmological applications; and (iii) repeat spectroscopic observations to enable the reverberation mapping of active galactic nuclei. Our simulations predict we will be able to classify transients down to r = 22.5 magnitudes (AB) and, over five years of 4MOST operations, obtain spectra for up to 30 000 live transients to redshift z 0.5, measure redshifts for up to 50 000 transient host galaxies to z 1 and monitor around 700 active galactic nuclei to z 2.5....