Black holes are some of the most exotic astronomical objects in the universe, and are accordingly the focus of much speculation and study. By definition invisible, they are only detectable when interacting with their environment, either by bending surrounding light or gravitationally capturing enough matter to power other processes whose radiation we can detect. Depending on the spin of the black hole (BH), its mass-to-energy conversion efficiency can be over 50 times higher than that of nuclear fusion. The gravitational energy liberated via this accretion process powers many phenomena around BHs and significantly impacts the surrounding regions. Because many of these phenomena involve complicated geometries, high-energy electromagnetic fields and strong gravity, they are still not well understood. By observing and modeling the radiation originating from the more extreme regions of BHs and their associated structures in our Galaxy, the planned ITN network intends to reconstruct a more complete picture of the complex physics involved in the accretion process through observations from the radio band to the X-rays and through theoretical work. Training on these subjects will be provided through summer schools on multi-wavelength astronomical techniques and on black hole astrophysics and through dedicated PhD projects.