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Over the last 50 years, astronomers have been intrigued by enormously energetic objects called Active Galactic Nuclei (AGN), a violent phenomenon occurring in the nuclei, or central regions, of some galaxies with intensities and durations which cannot easily be explained by stars, thus providing some of the first circumstantial evidence for theoretically-predicted supermassive black holes. Despite their intriguing properties they were largely viewed as interesting but unimportant freaks in the broader study of galaxy formation and evolution, leading astronomers studying the properties of galaxies to exclude the small fraction of galaxies with active centres as irritating aberrations. Here I describe the discovery of AGN and the variety of classifications that followed; I describe some features of unifying models of the central engine that attempt to explain the varied properties of different AGN classes that give rise to the classification. The search for supermassive black holes in AGN and non-active galaxies is discussed along with the developing realisation that all galaxies with significant bulge components might harbour dormant supermassive black holes as remnants of a past adolescent period of quasar activity and therefore posses the potential to be re-triggered into activity under the right conditions, making nuclear activity an integral part of galaxy formation and evolution.