Emperor Xuanzong of Tang, personal name Li Longji, was an emperor of the Tang dynasty of China, reigning from 712 to 756 CE. His reign of 44 years was the longest during the Tang dynasty. Through two palace coups, he seized the throne and inherited an empire still in its golden age. He was initially assisted by capable chancellors like Yao Chong, Song Jing and Zhang Yue who were already serving as government officials before Xuanzong ascended the throne. However, under Emperor Xuanzong, the empire reached its turning point and went into sharp decline and near collapse, due to numerous political missteps throughout his long reign, such as over-trusting Li Linfu, Yang Guozhong and An Lushan, with Tang's golden age ending in the An Lushan Rebellion.

Throughout Chinese history, "Emperor" was the superlative title held by the monarchs who ruled various imperial dynasties. In traditional Chinese political theory, the emperor was the "Son of Heaven", an autocrat with the divine mandate right to rule all under Heaven. Emperors were worshiped posthumously under an imperial cult. The lineage of emperors descended from a paternal family line constituted a dynasty, and succession in most cases theoretically followed agnatic primogeniture.