asp, and even fundamental quantities such as space and time are severely disrupted by their presence.  Yet, despite their fantastical nature, astronomers have compiled significant evidence that black holes are actually quite common and are lying at the centers of almost all massive galaxies. Therefore, black holes are no longer the theoretical subjects of mathematical physicists; they are now known to be crucial to our understanding of how galaxies and other structures in the Universe formed and evolved. This talk will provide an overview of our understanding of black holes in the observable universe, and outline how astrophysicists are using them to probe some of the deepest questions in the cosmos.