Among the city's Roman monuments are the Porta Nigra (early 4th cent.), an imposing and well-preserved fortified gate; an amphitheater (c.100), which can seat about 25,000 persons; ruins of the imperial baths (4th cent.); and the basilica (probably built in the early 4th cent.; now a church). Trier also has a Romanesque cathedral, built (11th–12th cent.) around a 4th-century nucleus and containing the Holy Coat of Treves (supposed to be the seamless coat of Jesus). Other noteworthy buildings include the Gothic Church of Our Lady (13th cent.; Ger. Liebfrauenkirche); the baroque electoral palace (17th–18th cent.); and the baroque Church of St. Paulinus (1732–54; designed by B. Neumann). The rare exhibitions (e.g., in 1844, 1891, 1933, and 1959) of the Holy Coat of Treves have been the occasions of large pilgrimages. The remains of St. Matthew are preserved in a shrine in the pilgrimage church of St. Matthew (built in the 12th cent. around an earlier Benedictine monastery). Trier also has a theological seminary, a school of viticulture, and several museums, including one in the house where Karl Marx was born (1818).

One of the oldest cities in Germany, Trier has played an important role in its history since Roman times and retains many Roman monuments. Founded by Augustus c.15 B.C., the city was made (1st cent.) the capital of the Roman province of Belgica and later became (3d cent.) the capital of the prefecture of Gaul; it was named after the Treveri, a people of E Gaul. Under the Roman Empire Trier attained a population of c.50,000 and became a major commercial center, with a large wine trade. It was a frequent residence of the Western emperors from c.295 until its capture (early 5th cent.) by the Franks.

The city was made an episcopal see in the 4th cent. and an archiepiscopal see c.815. Under the rule of the archbishops, Trier flourished as a commercial and cultural center. Trier was the seat of a university from 1473 until it was occupied by the French in 1797. The archbishopric of Trier was secularized and was formally ceded to France in 1801 by the Treaty of Lunéville. At the Congress of Vienna the city and most of the archbishopric were awarded (1815) to Prussia; territory E of the Rhine was given to Nassau and, with Nassau, passed to Prussia in 1866. Trier again became an episcopal see in 1821. It was occupied by France after World War I and suffered considerable damage in World War II.