Supermassive black holes at the centre of galaxies can accrete matter from their surroundings, yielding the class of Active Galactic Nuclei (AGN). These AGN monsters, which expel winds and relativistic jets, are known to play a key role in the formation of the Universe that we see today and so identifying these objects is of paramount importance. While in nearby galaxies this is relatively easy, the task becomes more difficult in distant galaxies, where circumnuclear dust can obscure our view of these monsters. In this thesis, we use radio observations, which are not affected by this obscuration, in order to identify these AGN. In particular we use a technique called Very Long Baseline Interferometry (VLBI) which uses multiple radio telescopes spread across continents to provide the sharpest possible view on the sky. This method effectively isolates compact radio cores which can only be attributed to AGN. This thesis pioneered this technique, provided new calibration methods, and almost tripled the number of radio-detected AGN in a famous deep-field. We compared VLBI to other well-known AGN selection techniques, revealing that VLBI provides an invaluable contribution to our understanding of the AGN content of the Universe. In this thesis, we investigated another measure of AGN activity, namely variability. A radio source that varies in brightness rapidly is most likely due to an AGN. We provided a first insight into the faintest variable radio sources, revealing that only a small percentage of radio sources are variable AGN.