Chang'an  listen (help·info) (simplified Chinese: 长安; traditional Chinese: 長安; pinyin: Cháng'ān; Wade–Giles: Ch'ang-an) is an ancient capital of more than ten dynasties in Chinese history, today known as Xi'an. Chang'an literally means "Perpetual Peace" in Classical Chinese. During the short-lived Xin Dynasty, the city was renamed "Constant Peace" (Chinese: 常安; pinyin: Cháng'ān); yet after its fall in AD 23, the old name was restored. By the time of the Ming Dynasty, the name was again changed to Xi'an, meaning "Western Peace", which has remained its name to the present day.

Chang'an had been settled since the Neolithic times, during which the Yangshao Culture established in Banpo in the city's suburb. Also in the northern vicinity of the modern Xi'an, the tumulus ruler Qin Shi Huang of Qin Dynasty held his imperial court, and constructed his massive mausoleum guarded by the famed Terracotta Army.

From its capital at Xianyang, the Qin dynasty ruled a larger area than either of the preceding dynasties. The imperial city of Chang'an during the Han Dynasty was located in northwest of today's Xi'an. During the Tang Dynasty, the area to be known as Chang'an included the area inside the Ming Xi'an fortification, plus some small areas to its east and west, and a major part of its southern suburbs. The Tang Chang'an hence, was 8 times the size of the Ming Xi'an, which was reconstructed upon the premise of the former imperial quarter of the Sui and Tang city. During its heyday, Chang'an was one of the largest and most populous cities in the world. Around AD 750, Chang'an was called a "million people's city" in Chinese records, while modern estimates put it at around 800,000–1,000,000 within city walls. According to the census in 742 recorded in the New Book of Tang, 362,921 families with 1,960,188 persons were counted in Jingzhao Fu (京兆府), the metropolitan area including small cities in the vicinity.