A large fraction of sources in the X-ray sky are variable on time scales from sub-seconds up to decades–X-ray astronomy has thus always been “time-domain astronomy”. By virtue of its survey strategy, eROSITA will provide variability information on time scales of hours up to years during the entire survey, with higher frequency visits being possible for sources at higher ecliptical latitudes. The aim of this project is to provide the scientific infrastructure for research on transient sources to be used in projects P3, P4, and P5, in conjunction with research that is complementary to that provided in these projects.