The population of Wales doubled from 587,000 in 1801 to 1,163,000 in 1851 and had reached 2,421,000 by 1911. Most of the increase came in the coal mining districts especially Glamorganshire, which grew from 71,000 in 1801 to 232,000 in 1851 and 1,122,000 in 1911. Part of this increase can be attributed to the demographic transition seen in most industrialising countries during the Industrial Revolution, as death-rates dropped and birth-rates remained steady. However, there was also a large-scale migration of people into Wales during the industrial revolution. The English were the most numerous group, but there were also considerable numbers of Irish and smaller numbers of other ethnic groups, including Italians migrated to South Wales. Wales received other immigration from various parts of the British Commonwealth of Nations in the 20th century, and African-Caribbean and Asian communities add to the ethno-cultural mix, particularly in urban Wales. Many of these self-identify as Welsh.

1900-1914

The modern history of Wales starts in the 19th century when South Wales became heavily industrialised with ironworks; this, along with the spread of coal mining to the Cynon and Rhondda valleys from the 1840s, led to an increase in population. The social effects of industrialisation resulted in armed uprisings against the mainly English owners. Socialism developed in South Wales in the latter part of the century, accompanied by the increasing politicisation of religious Nonconformism. The first Labour MP, Keir Hardie, was elected as junior member for the Welsh constituency of Merthyr Tydfil and Aberdare in 1900.

The first decade of the 20th century was the period of the coal boom in South Wales, when population growth exceeded 20 per cent. Demographic changes affected the language frontier; the proportion of Welsh speakers in the Rhondda valley fell from 64 per cent in 1901 to 55 per cent ten years later, and similar trends were evident elsewhere in South Wales.

Kenneth O. Morgan argues that the 1850-1914 era:

was a story of growing political democracy with the hegemony of the Liberals in national and local government, of an increasingly thriving economy in the valleys of south Wales, the world’s dominant coal-exporting area with massive ports at Cardiff and Barry, an increasingly buoyant literature and a revival in the eisteddfod, and of much vitality in the nonconformist chapels especially after the short-lived impetus from the ‘great revival’, Y Diwygiad Mawr, of 1904–5. Overall, there was a pervasive sense of strong national identity, with a national museum, a national library and a national university as its vanguard.

1914-1945

The world wars and interwar period were hard times for Wales, in terms of the Faltering economy of antiwar losses, and a deep sense of insecurity. Men eagerly volunteered for war service. Morgan argues:

1914–45, there was an abrupt and corrosive change. First World War was an ordeal not only for the loss of life, but for the startling collapse of economic life in south Wales and much resultant social deprivation. The war also saw the downfall of Lloyd George’s Liberal Party and the concordant national revival of pre-1914. The Welsh-speaking world went into retreat, though there was powerful compensation in the proliferating Anglo-Welsh poetry and prose of Dylan Thomas and many others. The Second World War brought more upheaval, though also the birth of a revival of the south Wales economy through the stimulus provided by the Board of Trade.

The Labour Party replaced the Liberals as the dominant party in Wales after the First World War, particularly in the industrial valleys of South Wales. Plaid Cymru was formed in 1925 but initially its growth was slow and it gained few votes at parliamentary elections.

Since 1945

Horgan characterizes the recent period as:

one of broad renewal, political resurgence under the Labour Party and unions, a marked revival of economic growth, with much great material affluence and social welfare. The final period saw a phenomenon little in evidence before 1939, a strong movement towards political nationalism, some success for Plaid Cymru and, after the Kilbrandon Commission, a major attempt to pass Welsh devolution.

The coal industry steadily declined after 1945. By the early 1990s there was only one deep pit still working in Wales. There was a similar catastrophic decline in the steel industry (the steel crisis), and the Welsh economy, like that of other developed societies, became increasingly based on the expanding service sector.

In May 1997, a Labour government was elected with a promise of creating devolved institutions in Scotland and Wales. In late 1997 a referendum was held on the issue which resulted a "yes" vote. The Welsh Assembly was set up in 1999 (as a consequence of the Government of Wales Act 1998) and possesses the power to determine how the government budget for Wales is spent and administered.

The results of the 2001 Census showed an increase in the number of Welsh speakers to 21% of the population aged 3 and older, compared with 18.7% in 1991 and 19.0% in 1981. This compares with a pattern of steady decline indicated by census results during the 20th century. The 2011 census showed that decline to have resumed. Though still higher than in 1991, the number of people aged 3 and over able to speak Welsh in Wales decreased from 582,000 (20.8 per cent) in 2001, to 562,000 (19.0 per cent) in 2011.

The Government of Wales Act 2006 (c 32) is an Act of the Parliament of the United Kingdom that reformed the National Assembly for Wales and allows further powers to be granted to it more easily. The Act creates a system of government with a separate executive drawn from and accountable to the legislature. Following a successful referendum in 2011 on extending the law making powers of the National Assembly it is now able to make laws, known as Acts of the Assembly, on all matters in devolved subject areas, without needing the UK Parliament's agreement. In the 2016 referendum, Wales joined England in endorsing Brexit and rejecting membership in the European Union.