  This paper briefly reviews past, current, and future efforts to image black
holes in the radio regime. Black holes seem like mystical objects, but they are
an integral part of current astrophysics and are at the center of attempts to
unify quantum physics and general relativity. Yet, nobody has ever seen a black
hole. What do they look like? Initially, this question seemed more of an
academic nature. However, this has changed over the past two decades.
Observations and theoretical considerations suggest that the supermassive black
hole, Sgr A*, in the center of our Milky Way is surrounded by a compact, foggy
emission region radiating at and above 230 GHz. It has been predicted that the
event horizon of Sgr A* should cast its shadow onto that emission region, which
could be detectable with a global VLBI array of radio telescopes. In contrast
to earlier pictures of black holes, that dark feature is not supposed to be due
to a hole in the accretion flow, but would represent a true negative image of
the event horizon. Currently, the global Event Horizon Telescope consortium is
attempting to make such an image. In the future those images could be improved
by adding more telescopes to the array, in particular at high sites in Africa.
Ultimately, a space array at THz frequencies, the Event Horizon Imager, could
produce much more detailed images of black holes. In combination with numerical
simulations and precise measurements of the orbits of stars - ideally also of
pulsars - these images will allow us to study black holes with unprecedented
precision.
