Nonconformity was a major religious movement in Wales from the 18th to the 20th centuries. The Welsh Methodist revival of the 18th century was one of the most significant religious and social movements in the modern history of Wales. The revival began within the Church of England in Wales, partly as a reaction to the neglect generally felt in Wales at the hands of absentee bishops and clergy. For two generations from the 1730s onwards the main Methodist leaders such as Howell Harris, Daniel Rowland and William Williams Pantycelyn remained within the Church of England, but the Welsh revival differed from the Methodist revival in England in that its theology was Calvinist rather than Arminian. Methodists in Wales gradually built up their own networks, structures, and meeting houses (or chapels), which led, at the instigation of Thomas Charles, to the secession of 1811 and the formal establishment of the Calvinistic Methodist denomination in 1823 (which assumed as an alternative title the name Presbyterian Church of Wales in 1928).

The 18th-century revival also influenced the older nonconformist churches, or dissenters – the Baptists and the Congregationalists – who in turn also experienced growth and renewal. As a result, by the middle of the 19th century, Wales was predominantly a nonconformist country.

Starting in rural areas and small market towns, the 19th century was the golden age of Welsh nonconformity. Some small settlements, such as Carmel, Nebo and Sardis, grew around a chapel or meeting house and were named after it. Some of these settlements, such as Bethesda, Gwynedd, became large settlements.

In the era of rapid industrialisation after the Napoleonic Wars, nonconformity became firmly established in the new industrial settlements of South Wales as migrants from the rural counties brought their religious affiliations with them. In places such as Merthyr Tydfil, Aberdare, Llanelli and Neath, Nonconformity grew alongside industry and by the 1880s these towns were regarded as "citadels of dissent", with their ministers and deacons having a powerful role among a new middle class elite which dominated public life. The ministers also had considerable influence within working class networks which, in part, reflected their own social origins. The Religious Census of 1851 showed that 80% of those who attended a place of worship on Census Sunday in Wales were Nonconformists, even though the large proportion of the population, even in Wales, who were not recorded in the census, suggested that the influence of religion within society was far more tenuous than first appearances suggested.

In 1859 there was another popular revival, which began in north Cardiganshire and eventually affected most of Wales. It was primarily, however, a Welsh language phenomenon, and its influence was, in the main, a transient one.^{[citation needed]}

Nevertheless, by the 1880s, Nonconformity was approaching its golden age. Chapel buildings were increasingly grandiose, in contrast to the austerity of the earlier meeting houses of the revivalist period. Professional architects were engaged to design elaborate structures, especially in larger towns such as Aberystwyth, Caernarfon, Carmarthen and Swansea, as well as in the developing townships of the industrial valleys.^{[citation needed]}

In the nineteenth century, Welsh nonconformity, within which the Calvinistic Methodists were the largest denomination, was strongly connected to the Welsh language, though not exclusively so. Anglicanism, in turn, was often associated with the English language and landed gentry. The growth in Welsh political radicalism in the 19th century was closely tied to the surge of Nonconformism. The wealthier and more established farm owners and the middle class spoke English. But the rapid industrializing and growth of coal and iron brought many poor farmers out of agriculture into much better paying industrial jobs, thus providing a better financial base for the Nonconformists.

By the 1880s, over 350,000 men and women were officially members of one of the four main Nonconformist denominations, and there were also larger numbers of "adherents" who attended services and other chapel-based activities but were not officially recorded as members. While there were some differences between the denominations, such as the Baptists' insistence on total immersion, these were less significant by the later 19th century than previously. In political terms, especially after the more conservative Calvinistic Methodists became more involved in political activity in the wake of the 1847 Reports of the Commissioners of Inquiry into the State of Education in Wales and the 1868 General Election, the chapels spoke largely as one in their opposition to the social and political dominance of the clergy and landed gentry, particularly in rural areas. The relative absence of these influences in the industrial valleys made Nonconformity even more powerful, especially given that many of the leading coal owners of the late 19th century were Welsh-speaking Nonconformists. It has been said that the chapels "formed almost a kind of unofficial established religion".