In Britain and Ireland, a workhouse (Welsh: tloty, lit. "poor-house") was a total institution where those unable to support themselves financially were offered accommodation and employment. In Scotland, they were usually known as poorhouses. The earliest known use of the term workhouse is from 1631, in an account by the mayor of Abingdon reporting that "we have erected within our borough a workhouse to set poorer people to work".

The origins of the workhouse can be traced to the Statute of Cambridge 1388, which attempted to address the labour shortages following the Black Death in England by restricting the movement of labourers, and ultimately led to the state becoming responsible for the support of the poor. However, mass unemployment following the end of the Napoleonic Wars in 1815, the introduction of new technology to replace agricultural workers in particular, and a series of bad harvests, meant that by the early 1830s the established system of poor relief was proving to be unsustainable. The New Poor Law of 1834 attempted to reverse the economic trend by discouraging the provision of relief to anyone who refused to enter a workhouse. Some Poor Law authorities hoped to run workhouses at a profit by utilising the free labour of their inmates. Most were employed on tasks such as breaking stones, crushing bones to produce fertiliser, or picking oakum using a large metal nail known as a spike.

As the 19th century wore on, workhouses increasingly became refuges for the elderly, infirm, and sick rather than the able-bodied poor, and in 1929 legislation was passed to allow local authorities to take over workhouse infirmaries as municipal hospitals. Although workhouses were formally abolished by the same legislation in 1930, many continued under their new appellation of Public Assistance Institutions under the control of local authorities. It was not until the introduction of the National Assistance Act 1948 (11 & 12 Geo. 6. c. 29) that the last vestiges of the Poor Law finally disappeared, and with them the workhouses.

The Statute of Cambridge 1388 was an attempt to address the labour shortage caused by the Black Death, a devastating pandemic that killed about one-third of England's population. The new law fixed wages and restricted the movement of labourers, as it was anticipated that if they were allowed to leave their parishes for higher-paid work elsewhere then wages would inevitably rise. According to historian Derek Fraser, the fear of social disorder following the plague ultimately resulted in the state, and not a "personal Christian charity", becoming responsible for the support of the poor. The resulting laws against vagrancy were the origins of state-funded relief for the poor. From the 16th century onwards a distinction was legally enshrined between those who were willing to work but could not, and those who were able to work but would not: between "the genuinely unemployed and the idler". Supporting the destitute was a problem exacerbated by King Henry VIII's Dissolution of the Monasteries, which began in 1536. They had been a significant source of charitable relief, and provided a good deal of direct and indirect employment. The Poor Act 1575 went on to establish the principle that if the able-bodied poor needed support, they had to work for it.

The Poor Relief Act 1601 made parishes legally responsible for the care of those within their boundaries who, through age or infirmity, were unable to work. The Act essentially classified the poor into one of three groups. It proposed that the able-bodied be offered work in a house of correction (the precursor of the workhouse), where the "persistent idler" was to be punished. It also proposed the construction of housing for the impotent poor, the old and the infirm, although most assistance was granted through a form of poor relief known as outdoor relief – money, food, or other necessities given to those living in their own homes, funded by a local tax on the property of the wealthiest in the parish.

The workhouse system evolved in the 17th century, allowing parishes to reduce the cost to ratepayers of providing poor relief. The first authoritative figure for numbers of workhouses comes in the next century from The Abstract of Returns made by the Overseers of the Poor, which was drawn up following a government survey in 1776. It put the number of parish workhouses in England and Wales at more than 1800 (about one parish in seven), with a total capacity of more than 90,000 places. This growth in the number of workhouses was prompted by the Workhouse Test Act 1723; by obliging anyone seeking poor relief to enter a workhouse and undertake a set amount of work, usually for no pay (a system called indoor relief), the Act helped prevent irresponsible claims on a parish's poor rate.

The growth was also bolstered by the Relief of the Poor Act 1782, proposed by Thomas Gilbert. Gilbert's Act was intended to allow parishes to share the cost of poor relief by joining together to form unions, known as Gilbert Unions, to build and maintain even larger workhouses to accommodate the elderly and infirm. The able-bodied poor were instead either given outdoor relief or found employment locally. Relatively few Gilbert Unions were set up, but the supplementing of inadequate wages under the Speenhamland system did become established towards the end of the 18th century. So keen were some Poor Law authorities to cut costs wherever possible that cases were reported of husbands being forced to sell their wives, to avoid them becoming a financial burden on the parish. In one such case in 1814 the wife and child of Henry Cook, who were living in Effingham workhouse, were sold at Croydon market for one shilling (5p); the parish paid for the cost of the journey and a "wedding dinner".

By the 1830s most parishes had at least one workhouse, but many were badly managed. In his 1797 work, The State of the Poor, Sir Frederick Eden, wrote: