Black holes are defined as a region in spacetime where gravity is so strong that particles and electromagnetic radiation cannot escape. According to their mass, they are classified into three types: stellar-mass black holes, intermediate-mass black holes, and supermassive black holes. This entry describes how to weight and detect these three types of black holes, summarizes key research findings such as the universality of black hole accretion and black hole-galaxy co-evolution, and gives an outlook to what the next generation of observational facilities will provide.

10 pages, 5 figures, published entry in Encyclopedia of Astrobiology