Emperor Xuanzong of Tang ([ɕwǎn.tsʊ́ŋ]; 8 September 685 – 3 May 762), personal name Li Longji, was an emperor of the Tang dynasty of China, reigning from 712 to 756. 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 chancellors Li Linfu, Yang Guozhong and general An Lushan, with Tang's golden age ending in the An Lushan rebellion.

Li Longji was born at the Tang dynasty eastern capital Luoyang in 685, during the first reign of his father Emperor Ruizong (Li Dan) – but at that time, Emperor Ruizong's mother Empress Dowager Wu (later known as Wu Zetian), not Emperor Ruizong, was in actual control of power as empress dowager and regent. Li Longji was the third son of Emperor Ruizong, and his mother was Emperor Ruizong's concubine Consort Dou, who was highly ranked in the bureaucracy. In 687, as the emperor's son, he was created the Prince of Chu. It was said that he was handsome, with an appearance that was "majestic and beautiful", and was talented in music. He had two older brothers – Li Chengqi, born of Emperor Ruizong's wife Empress Liu, and Li Chengyi (李成義), as well as three younger brothers – Li Longfan (李隆範), Li Longye (李隆業), and Li Longti (李隆悌). He had two full younger sisters, Princess Jinxian (金仙公主) and Princess Yuzhen (玉真公主), who later became Taoist nuns.

In 690, Empress Dowager Wu had her son Emperor Ruizong yield the throne to her, and she took the throne as empress regnant of a new Zhou dynasty, interrupting Tang. She imposed upon Ruizong and his family the surname Wu (武) to match hers. In 692, Li Longji and his brothers were allowed to have residences outside the palace and were given staffs at their mansions. In 693, both his mother Consort Dou and Li Dan's wife Crown Princess Liu (Li Dan's having been reduced to crown prince rank at that point) were killed by Wu Zetian inside the palace after Wu Zetian's lady-in-waiting Wei Tuan'er (韋團兒) falsely accused them of using witchcraft against Wu Zetian – and not even their bodies were recovered. Subsequently, all of Li Dan's sons were reduced in title, and Li Longji's title was reduced to Prince of Linzi. He and his brothers, along with their cousins Li Guangshun (李光順) the Prince of Yifeng, Li Shouli the Prince of Yong, and Li Shouyi (李守義) the Prince of Yong'an (the sons of their uncle Li Xián, formerly crown prince but who had been forced by Wu Zetian to commit suicide in 684), were kept inside the palace and not allowed to have contact with outsiders until 699, when they were allowed to leave the palace and take up residences outside.

In 705, Wu Zetian was overthrown in a coup (Shenlong Coup [zh]), and Li Longji's uncle Li Xiǎn (note different person than the father of Li Guangshun, Li Shouli, and Li Shouyi), who was at that time crown prince (Li Dan having yielded that title to him in 698 and taken the title of Prince of Xiang), who had been briefly emperor prior to Li Dan, returned to the throne (as Emperor Zhongzong). Li Longji was made the deputy minister of military supplies (衛尉少卿, Weiwei Shaoqing). In 708, he was made the secretary general of Lu Prefecture (潞州, roughly modern Changzhi, Shanxi). In 710, he was recalled to the capital Chang'an to attend to Emperor Zhongzong when Emperor Zhongzong was sacrificing to heaven and earth. Meanwhile, sorcerers engaged by Emperor Zhongzong believed that there was an aura of an emperor at the area of Chang'an where the mansions Li Longji and his uncles were, and Emperor Zhongzong tried to fulfill the vision by visiting Li Longji's mansion and attending a feast there. While Li Longji was back in Chang'an, he spent time to cultivate relationships with imperial guard commanders, as he believed that Emperor Zhongzong's powerful wife Empress Wei, who was in charge of government affairs, would bring harm to the Tang dynasty.

In summer 710, Emperor Zhongzong died suddenly—a death that traditional historians believed to be a poisoning by Empress Wei and her daughter Li Guo'er the Princess Anle so that Empress Wei could become "emperor" like Wu Zetian and Li Guo'er could become crown princess. For the time being, Emperor Zhongzong's son by a concubine, Li Chongmao the Prince of Wen, was named emperor (as Emperor Shang), but Empress Wei retained actual power as empress dowager and regent. Originally, Empress Dowager Wei's clan members, along with Zong, Li Guo'er's husband Wu Yanxiu, and other officials Zhao Lüwen (趙履溫) and Ye Jingneng (葉靜能) were advising her to take the throne, like Wu Zetian did, and they also advised her to eliminate Li Dan and Princess Taiping. The official Cui Riyong leaked their plan to Li Longji. Li Longji responded by conspiring with Princess Taiping, Princess Taiping's son Xue Chongjian (薛崇簡), as well as several low-level officials close to him—Zhong Shaojing, Wang Chongye (王崇曄), Liu Youqiu, and Ma Sizong (麻嗣宗)—to act first. Meanwhile, Empress Wei's nephews Wei Bo (韋播) and Gao Song (高嵩), who had recently been put in command of imperial guards and who had tried to establish their authority by dealing with the guards harshly, had alienated the guards, and the guard officers Ge Fushun (葛福順), Chen Xuanli (陳玄禮), and Li Xianfu (李仙鳧) thereafter also joined the plot.

Without first informing Li Dan, the conspirators enacted their plan on 21 July 710, first killing Wei Bo, Gao, and Empress Wei's cousin Wei Gui (韋璿). They then began their attack on the palace, causing Empress Dowager Wei to flee to an imperial guard camp, where a guard beheaded her. Li Guo'er, Wu Yanxiu, and Lady Helou were killed as well. Li Longji soon slaughtered a number of officials in Empress Dowager's faction as well as her clan, while displaying Empress Dowager Wei's body on the street. At the urging of Princess Taiping, Li Longji, and Li Longji's brother Li Chengqi, Li Dan soon took the throne from Li Chongmao and again became emperor (as Emperor Ruizong).

Emperor Ruizong once again ascended to the throne, which is different from the first time he was a puppet. This time he truly mastered the power and quickly used it handily. Emperor Ruizong was immediately faced with the issue of whom to make crown prince—as Li Chengqi, as the oldest son overall and the oldest son of his wife, was the appropriate heir under Confucian principles of succession, but Li Longji had been the one whose accomplishments had allowed him to retake the throne. He hesitated. Li Chengqi declined consideration to be crown prince—stating to his father:

If the state were secure, then consideration should be first given to the oldest son of the wife. If the state were in danger, then consideration should be first given for achievement. If you did not follow this principle, the people of the entire empire will be disappointed. I would rather die than to be placed above the Prince of Ping [(i.e., Li Longji, whose title had been changed to Prince of Ping by this point)].