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Gravity is successfully described by Einstein’s theory of general relativity, governing the structure of our entire universe. Yet gravity remains the least understood of all forces in nature, e.g., resisting unification with quantum physics. One of the most fundamental pre-dictions of general relativity are black holes. Their defining feature is the event horizon, the surface that even light cannot escape and where time and space exchange their nature. How-ever, while there are many convincing black hole candidates in the universe, there is no exper-imental proof for the existence of an event horizon yet. So, does general relativity really hold in its most extreme limit? Do BHs exist or are alternatives needed? The best place to test this is in the center of our own Milky Way. Here a compact radio source with a mass of 4 Million times the mass of the sun, marks the central point of our Galaxy, providing by the far the best evidence for the existence of black holes. Very long baseline radio observations are now prob-ing the smallest scales of this source, making it possible to image the shadow of the event horizon of a black hole for the very first time. Moreover, with the help of advanced numerical general relativistic magneto-hydrodynamic simulations emission and appearance of the source can be successfully modeled almost from first principles. Hence, the Galactic center provides today a unique laboratory for astrophysics and general relativity.