Structural evolution in synchrotron sources over the mass-scale range, from VLBI towards the SKA era

Very Long Baseline Interferometry (VLBI) provides milliarcsecond scale angular resolution imaging. This proves suitable to monitor structural evolution on AU scales in our Galaxy and on parsec scales in extragalactic sources. Given the high brightness temperatures required for detection on such small angular scales, VLBI observations are typically focused on non-thermal sources like relativistic jets and shocked emission regions in the environment of compact sources.  I will present a few selected results over various mass scales, including novae, such as 2021 RS Oph and the upcoming T CrB outburst; binary neutron star mergers, like the famous GW 170817 event; jets launched in the vicinity of supermassive black hole, such as the radio galaxy M87.  Moving from these results and the associated challenges, I will describe the impact of the SKA for VLBI studies for transients and radio loud AGNs.