Our understanding of space and time is probed to its depths by black holes. These objects, which appear as a natural consequence of general relativity, provide a powerful tool able to examine macroscopic and microscopic properties of the universe. I shall present the basic concepts of black hole?s theory, as well as a description of the astronomical sites where black holes are suspected to lie, namely binary X?ray sources and galactic nuclei. Spectacular high energy phenomena already predicted by theorerical models, such as the destruction of stars by giant black holes or high-velocity black holes wandering in space, are now currently observed by space telescopes and provide a better understanding of the most fascinating objects of modern astrophysics.