Black holes lie at the heart of some of the most fascinating astrophysical phenomena. IAU Symposium 324 marked the 100th anniversary of Schwarzschild's solution of Einstein's field equations predicting the existence of black holes. Our understanding of black holes has come an impressively long way since then, with the last major discovery being coalescing black holes producing gravitational waves, also predicted in 1916. In this volume, observational and theoretical experts discuss the current state-of-the-art in the astrophysics of black-hole systems and their exploitation in testing fundamental theories of physics. Topics span a wide range and include a historical review, the similarity and diversity of black hole systems, gamma ray bursts, tidal disruption events, active galactic nuclei, black hole systems as multi-messenger sources, and the opening of new observational horizons. This fresh review is especially useful to researchers and graduate students engaged in these exciting fields.