Gnomoniopsis castaneae is an emerging fungal pathogen currently scored as the major nut rot agent on chestnut, although it is also associated with cankers on both chestnut and hazelnut, as well as with necrosis on chestnut galls and leaves. Described for the first time in 2012, G. castaneae has been reported in several countries across Europe, Asia and Australasia, often in relation to severe outbreaks. The goal of this review is to provide a comprehensive summary of the state of the art about G. castaneae, highlighting the main results achieved by the research and stressing the most relevant knowledge gaps that still need to be filled. This overview includes topics encompassing the taxonomy of the fungal pathogen, its host range and geographic distribution, the symptomatology and the diagnostic methods available for its detection, its impact, biology, ecology and epidemiology. The main interactions between G. castaneae and other organisms are also discussed, as well as the possible control strategies. In these past few years, relevant progresses in the knowledge of G. castaneae have been achieved, yet the complexity of the challenges that this pathogen poses to chestnut growers and to the scientific community advocates for further advances.