Astrophysics has always been a captivating field, delving into the mysteries of the universe and providing glimpses into the workings of celestial objects. However, it is in the realm of high-energy astrophysics that the universe truly reveals its most extreme phenomena. From quasars to gamma-ray bursts, this subchapter explores the historical milestones that have unraveled the mysteries of high-energy astrophysics. In the early 20th century,...