Black holes are perhaps the most fascinating astrophysical objects we know about. An original prediction of the Einstein field equations, they are now thought to play an essential role in our image of the universe, from small to large scales. In this talk I will briefly review the main features of black hole physics and I will discuss the evidence supporting the existence of these objects. Then, I will extend on their multiple astrophysical manifestations, from stellar systems to the central engines a quasars.