The earliest inhabitants of Wales were from continental Europe, who migrated in several waves and who later adopted the culture of the Celts . Up to and during the Roman occupation of Britain, Wales was not a separate country, but all inhabitants of the British Isles spoke Celtic languages and were essentially of the same ethnic origin. The Romans occupied the whole of Wales, where they built roads and forts, mined gold and conducted commerce, but their interest in it was limited, because of the difficult geography and shortage of flat agricultural land. They established only one town in Wales - Caerwent (Venta Silurum). The Silures were the major tribe of south-east Wales. Their military leader, Caratacus (Caradoc), had joined them from another, defeated, tribe. Under his leadership, they defied the Romans for a period after the Claudian invasion, but eventually Caratacus was captured and taken to Rome, where his dignified bearing made such an impression on the people that his life was spared.

When Britain threw off the rule of Rome in AD 410, the various states within Wales were left self-governing. One of the reasons for the Roman withdrawal was the pressure put upon the empire's military resources by the incursion of barbarian tribes from the east. These tribes, including the Angles and Saxons, were unable to make inroads into Wales, but they gradually conquered the whole of England, leaving Wales cut off from her Celtic relations in Scotland, Cornwall, and Cumbria. Wales remained Christian, and the "age of the saints" (approximately 500-700 AD) was marked by the establishment of monastic settlements throughout the country, by religious leaders such as David, Illtud and Teilo. Wales was divided into a number of separate territories, and for a single man to rule the whole country at this period was rare, the first to do so being Rhodri Mawr, during the 9th century AD. Rhodri's grandson, Hywel Dda, succeeded in drawing up a standard legal system and brought peace to the country, but, on his death, his territories were once again divided.

A major difficulty in achieving national unity was the inheritance system practised in Wales. All sons received an equal share of their father's property (including illegitimate sons). Liberal as this policy was, it resulted in frequent internecine violence and the division of small territories into still smaller ones, so that, by the time of the Norman conquest of England in 1066, Wales was again fragmented. The princes of Gwynedd, in the north, were increasingly dominant. Owain Gwynedd, who died in 1170, had a strong hold on his principality, but, following his death, his sons squabbled and murdered one another. Out of the ensuing power struggle eventually arose the greatest of all Welsh leaders, Llywelyn ap Iorwerth, also known as Llywelyn Fawr or Llywelyn the Great, but internal strife again broke out after his death, culminating in the rise to power of his grandson, Llywelyn ap Gruffydd Llywelyn the Last. Partly as a result of Llywelyn's own recklessness, he gained the antagonism of King Edward I of England, who determined to complete the conquest of Wales. After Llywelyn's death in battle in 1282, only token resistance was offered by the surviving princes. King Edward's ring of impressive stone castles assisted the domination of Wales, and he crowned his achievement by giving the title Prince of Wales to his son and heir in 1301.

Wales became, effectively, part of England, even though its people spoke a different language and had a different culture. English kings paid lip service to their responsibilities by appointing a Council of Wales, sometimes presided over by the heir to the throne. This Council normally sat in Ludlow, now in England but at that time still part of the disputed border area. In 1400, a Welsh nobleman, Owain Glyn Dwr or Owen Glendower, revolted against King Henry IV of England, inflicted several military defeats, and succeeded in evading capture, but he did not have the strength to survive as a leader. However, his rebellion caused a great upsurge in Welsh identity and he was widely supported by Welsh people throughout the country. Some of his achievements included the first ever Welsh Parliament and plans for two universities. Subsequently, a Welshman, Henry Tudor, gained the throne as King Henry VII of England. Under his son, Henry VIII of England, the Act of Union of 1536 was passed, annexing Wales to England in legal terms, abolishing the Welsh legal system, and banning the Welsh language from any official role or status.

In later centuries, parts of Wales became heavily industrialised, and the social effects of industrialisation led to bitter social conflict between the Welsh workers and the English factory owners. During the 1830s there were two armed uprisings, in the new town of Merthyr Tudful in 1831, and in the Eastern Valleys in 1839, leading to the country becoming a hotbed of socialism, accompanied by the increasing politicisation of religious nonconformism. The first Labour MP, Keir Hardie, was elected for the Welsh constituency of Merthyr in 1900. In common with many European nations, the first movements for national autonomy began in the 1880s and 1890s with the formation of Cymru Fydd. Wales was officially de-annexed from England within the United Kingdom in 1955, with the term 'England' being replaced with England and Wales.

Nationalism only became a major issue during the twentieth century, with the political party, Plaid Cymru, winning its first Parliamentary seat in 1966. Largely as a result of this, devolution became the policy of the Labour party, and the National Assembly for Wales was eventually established in 1998, with power over public spending within the principality.