Observing the unobservable:  a multi-messenger view of black holes in the Event Horizon Telescope era

The last decade has witnessed some amazing developments in astrophysics and astroparticle physics, not least of which is the ability to directly image two supermassive black holes with global mm-VLBI. But how do these images connect to broader questions regarding the workings of accretion in general, or the launching of jets and particle acceleration, common to a wide variety of astrophysical phenomena?  After a brief review of the Event Horizon Telescope (EHT) project and the current state-of-the-art, I will describe how these single-frequency imaging projects are starting to expand in several directions:  frequency, dynamical range, size, timescale. I will also discuss how the combination of such images with multi-wavelength and multi-messenger data, together with recent advances in theoretical modelling, have the potential to tackle some of the hairiest problems relating to black holes and compact objects in general.