Lydia (Ancient Greek: Λυδία, romanized: Ludía; Latin: Lȳdia) was an Iron Age kingdom situated in the west of Asia Minor, in modern-day Turkey. Later, it became an important province of the Achaemenid Empire and then the Roman Empire. Its capital was Sardis.

At some point before 800 BC, the Lydian people achieved some sort of political cohesion, and existed as an independent kingdom by the 600s BC. At its greatest extent, during the 7th century BC, it covered all of western Anatolia. In 546 BC, it became a satrapy of the Achaemenid Empire, known as Sparda in Old Persian. In 133 BC, it became part of the Roman province of Asia.

Lydian coins, made of electrum, are among the oldest in existence, dated to around the 7th century BC.

Lydia is generally located east of ancient Ionia in the modern western Turkish provinces of Uşak, Manisa and inland Izmir.

The boundaries of historical Lydia varied across the centuries. It was bounded first by Mysia, Caria, Phrygia and coastal Ionia. Later, the military power of Alyattes and Croesus expanded Lydia, which, with its capital at Sardis, controlled all Asia Minor west of the River Halys, except Lycia. After the Persian conquest the River Maeander was regarded as its southern boundary, and during imperial Roman times Lydia comprised the country between Mysia and Caria on the one side and Phrygia and the Aegean Sea on the other.

The Lydian language was an Indo-European language in the Anatolian language family, related to Luwian and Hittite. Due to its fragmentary attestation, the meanings of many words are unknown but much of the grammar has been determined. Similar to other Anatolian languages, it featured extensive use of prefixes and grammatical particles to chain clauses together. Lydian had also undergone extensive syncope, leading to numerous consonant clusters atypical of most Indo-European languages. Lydian finally became extinct during the 1st century BC.

The Lydian language is usually not categorized as part of the Luwic subgroup, unlike the other nearby Anatolian languages Luwian, Carian, and Lycian.

Lydia's early history remains shrouded in obscurity. During the Late Bronze Age (1600 BC-1200 BC), the territory that later became Lydia overlapped with two kingdoms called Mira and Šeḫa, themselves part of a broader political entity called Arzawa. Like the other Arzawa Lands, these kingdoms had tumultuous relations with the Hittite Empire, acting both as allies, enemies, and vassals at various points in time.

By roughly 800 BC, the Lydian people appear to have established their presence and achieved some degree of political cohesion. However, precise dates and events are impossible to determine due to the absence of contemporary written records. The only firm evidence for this early period comes from the archaeological excavations at Sardis. Although certain literary accounts purport the existence of two early Lydian dynasties, namely the house of Atys - after whose son Lydus the Lydians were supposedly named - and the Heraclids, who allegedly ruled for twenty-two generations before 685 BC, these sources are steeped in mythology and lack historical credibility.

Lydia was an independent kingdom from an unknown time until 546 BC.

According to Herodotus, one of Lydus's descendants was Iardanus, with whom Heracles was in service at one time. Heracles had an affair with one of Iardanus' slave-girls and their son Alcaeus was the first of the Heraclid Dynasty said to have ruled Lydia for 22 generations starting with Agron. In the 8th century BC, Meles became the 21st and penultimate Heraclid king and the last was his son Candaules (died c. 687 BC).

Gyges is the first king whose existence is demonstrable from contemporary records. According to semi-mythical accounts of his reign, he was the son of a man named Dascylus and came to power by overthrowing King Candaules with the assistance of a Carian prince from Mylasa named Arselis. Gyges's rise to power happened in the context of a period of turmoil following the invasion of the Cimmerians, a nomadic people from the Pontic steppe who had invaded Western Asia, who around 675 BC destroyed the previous major power in Anatolia, the kingdom of Phrygia.

Gyges took advantage of the power vacuum created by the Cimmerian invasions to consolidate his kingdom and make it a military power, he contacted the Neo-Assyrian court by sending diplomats to Nineveh to seek help against the Cimmerian invasions, and he attacked the Ionian Greek cities of Miletus, Smyrna, and Colophon. Gyges's extensive alliances with the Carian dynasts allowed him to recruit Carian and Ionian Greek soldiers to send overseas to assist the Egyptian king Psamtik I of the city of Sais, with whom he had established contacts around 662 BC. With the help of these armed forces, Psamtik I united Egypt under his rule after eliminating the eleven other kinglets with whom he had been co-ruling Lower Egypt.

In 644 BC, Lydia faced a third attack by the Cimmerians, led by their king Lygdamis. This time, the Lydians were defeated, Sardis was sacked, and Gyges was killed.

Gyges was succeeded by his son Ardys, who resumed diplomatic activity with Assyria and would also have to face the Cimmerians. Ardys attacked the Ionian Greek city of Miletus and succeeded in capturing the city of Priene, after which Priene would remain under direct rule of the Lydian kingdom until its end.

Ardys's reign was short-lived, and in 637 BC, that is in Ardys's seventh regnal year, the Thracian Treres tribe who had migrated across the Thracian Bosporus and invaded Anatolia, under their king Kobos, and in alliance with the Cimmerians and the Lycians, attacked Lydia. They defeated the Lydians again and for a second time sacked the Lydian capital of Sardis, except for its citadel. It is probable that Ardys was killed during this Cimmerian attack.

Ardys was succeeded by his son, Sadyattes, who had an even more short-lived reign. Sadyattes died in 635 BC, and it is possible that, like his grandfather Gyges and maybe his father Ardys as well, he died fighting the Cimmerians.

Amidst extreme turmoil, Sadyattes was succeeded in 635 BC by his son Alyattes, who would transform Lydia into a powerful empire.

Soon after Alyattes's ascension and early during his reign, with Assyrian approval and in alliance with the Lydians, the Scythians under their king Madyes entered Anatolia, expelled the Treres from Asia Minor, and defeated the Cimmerians so that they no longer constituted a threat again, following which the Scythians extended their domination to Central Anatolia until they were themselves expelled by the Medes from Western Asia in the 590s BC. This final defeat of the Cimmerians was carried out by the joint forces of Madyes, whom Strabo credits with expelling the Treres and Cimmerians from Asia Minor, and of Alyattes, whom Herodotus and Polyaenus claim finally defeated the Cimmerians.

Alyattes turned towards Phrygia in the east, where extended Lydian rule eastwards to Phrygia. Alyattes continued his expansionist policy in the east, and of all the peoples to the west of the Halys River whom Herodotus claimed Alyattes's successor Croesus ruled over - the Lydians, Phrygians, Mysians, Mariandyni, Chalybes, Paphlagonians, Thyni and Bithyni Thracians, Carians, Ionians, Dorians, Aeolians, and Pamphylians - it is very likely that a number of these populations had already been conquered under Alyattes, and it is not impossible that the Lydians might have subjected Lycia, given that the Lycian coast would have been important for the Lydians because it was close to a trade route connecting the Aegean region, the Levant, and Cyprus.

Alyattes's eastern conquests brought the Lydian Empire in conflict in the 590s BC with the Medes, and a war broke out between the Median and Lydian Empires in 590 BC which was waged in eastern Anatolia lasted five years, until a solar eclipse occurred in 585 BC during a battle (hence called the Battle of the Eclipse) opposing the Lydian and Median armies, which both sides interpreted as an omen to end the war. The Babylonian king Nebuchadnezzar II and the king Syennesis of Cilicia acted as mediators in the ensuing peace treaty, which was sealed by the marriage of the Median king Cyaxares's son Astyages with Alyattes's daughter Aryenis, and the possible wedding of a daughter of Cyaxares with either Alyattes or with his son Croesus.

Alyattes died shortly after the Battle of the Eclipse, in 585 BC itself, following which Lydia faced a power struggle between his son Pantaleon, born from a Greek woman, and his other son Croesus, born from a Carian noblewoman, out of which the latter emerged successful.

Croesus brought Caria under the direct control of the Lydian Empire, and he subjugated all of mainland Ionia, Aeolis, and Doris, but he abandoned his plans of annexing the Greek city-states on the islands of the Aegean Sea and he instead concluded treaties of friendship with them, which might have helped him participate in the lucrative trade the Aegean Greeks carried out with Egypt at Naucratis. According to Herodotus, Croesus ruled over all the peoples to the west of the Halys River, although the actual border of his kingdom was further to the east of the Halys, at an undetermined point in eastern Anatolia.

Croesus continued the friendly relations with the Medes concluded between his father Alyattes and the Median king Cyaxares, and he continued these good relations with the Medes after he succeeded Alyattes and Astyages succeeded Cyaxares. And, under Croesus's rule, Lydia continued its good relations started by Gyges with the Saite Egyptian kingdom, then ruled by the pharaoh Amasis II. Croesus also established trade and diplomatic relations with the Neo-Babylonian Empire of Nabonidus, and he further increased his contacts with the Greeks on the European continent by establishing relations with the city-state of Sparta.

In 550 BC, Croesus's brother-in-law, the Median king Astyages, was overthrown by his own grandson, the Persian king Cyrus the Great, and Croesus responded by attacking Pteria, the capital of a Phrygian state vassal to the Lydians which might have attempted to declare its allegiance to the new Persian Empire of Cyrus. Cyrus retaliated by intervening in Cappadocia and defeated the Lydians at Pteria in a battle, and again at Thymbra before besieging and capturing the Lydian capital of Sardis, thus bringing an end to the rule of the Mermnad dynasty and to the Lydian Empire. Lydia would never regain its independence and would remain a part of various successive empires.

Although the dates for the battles of Pteria and Thymbra and of end of the Lydian empire have been traditionally fixed to 547 BC, more recent estimates suggest that Herodotus's account being unreliable chronologically concerning the fall of Lydia means that there are currently no ways of dating the end of the Lydian kingdom; theoretically, it may even have taken place after the fall of Babylon in 539 BC.

In 547 BC, the Lydian king Croesus besieged and captured the Persian city of Pteria in Cappadocia and enslaved its inhabitants. The Persian king Cyrus The Great marched with his army against the Lydians. The Battle of Pteria resulted in a stalemate, forcing the Lydians to retreat to their capital city of Sardis. Some months later the Persian and Lydian kings met at the Battle of Thymbra. Cyrus won and captured the capital city of Sardis by 546 BC. Lydia became a province (satrapy) of the Persian Empire.

Lydia remained a satrapy after Persia's conquest by the Macedonian king Alexander III (the Great) of Macedon.

When Alexander's empire ended after his death, Lydia was possessed by the major Asian diadoch dynasty, the Seleucids, and when it was unable to maintain its territory in Asia Minor, Lydia was acquired by the Attalid dynasty of Pergamum. Its last king avoided the spoils and ravage of a Roman war of conquest by leaving the realm by testament to the Roman Empire.

When the Romans entered the capital Sardis in 133 BC, Lydia, as the other western parts of the Attalid legacy, became part of the province of Asia, a very rich Roman province, worthy of a governor with the high rank of proconsul. The whole west of Asia Minor had Jewish colonies very early, and Christianity was also soon present there. Acts of the Apostles 16:14–15 mentions the baptism of a merchant woman called "Lydia" from Thyatira, known as Lydia of Thyatira, in what had once been the satrapy of Lydia. Christianity spread rapidly during the 3rd century AD, based on the nearby Exarchate of Ephesus.

Under the tetrarchy reform of Emperor Diocletian in 296 AD, Lydia was revived as the name of a separate Roman province, much smaller than the former satrapy, with its capital at Sardis.

Together with the provinces of Caria, Hellespontus, Lycia, Pamphylia, Phrygia prima and Phrygia secunda, Pisidia (all in modern Turkey) and the Insulae (Ionian islands, mostly in modern Greece), it formed the diocese (under a vicarius) of Asiana, which was part of the praetorian prefecture of Oriens, together with the dioceses Pontiana (most of the rest of Asia Minor), Oriens proper (mainly Syria), Aegyptus (Egypt) and Thraciae (on the Balkans, roughly Bulgaria).

Under the Byzantine emperor Heraclius (610–641), Lydia became part of Anatolikon, one of the original themata, and later of Thrakesion. Although the Seljuk Turks conquered most of the rest of Anatolia, forming the Sultanate of Ikonion (Konya), Lydia remained part of the Byzantine Empire. While the Venetians occupied Constantinople and Greece as a result of the Fourth Crusade, Lydia continued as a part of the Byzantine rump state called the Nicene Empire based at Nicaea until 1261.

Lydia was captured finally by Turkish beyliks, which were all absorbed by the Ottoman state in 1390. The area became part of the Ottoman Aidin Vilayet (province), and is now in the modern republic of Turkey.

According to Herodotus, the Lydians were the first people to use gold and silver coins and the first to establish retail shops in permanent locations. It is not known, however, whether Herodotus meant that the Lydians were the first to use coins of pure gold and pure silver or the first precious metal coins in general. Despite this ambiguity, this statement of Herodotus is one of the pieces of evidence most often cited on behalf of the argument that Lydians invented coinage, at least in the West, although the first coins (under Alyattes I, reigned c.591–c.560 BC) were neither gold nor silver but an alloy of the two called electrum.

The dating of these first stamped coins is one of the most frequently debated topics of ancient numismatics, with dates ranging from 700 BC to 550 BC, but the most common opinion is that they were minted at or near the beginning of the reign of King Alyattes (sometimes referred to incorrectly as Alyattes II). The first coins were made of electrum, an alloy of gold and silver that occurs naturally but that was further debased by the Lydians with added silver and copper.