Sanitary districts were established in England and Wales in 1872 and in Ireland in 1878. The districts were of two types, based on existing structures:

Each district was governed by a local board of health, which was responsible for various public health matters such as providing clean drinking water, sewers, street cleaning, and clearing slum housing.

In England and Wales both rural and urban sanitary districts were replaced under by the Local Government Act 1894 (56 & 57 Vict. c. 73) by the more general rural districts and urban districts. A similar reform was carried out in Ireland in 1899 under the Local Government (Ireland) Act 1898.

Sanitary districts were formed under the terms of the Public Health Act 1872 (35 & 36 Vict. c. 79). Instead of creating new bodies, existing authorities were given additional responsibilities. The sanitary districts were created on 10 August 1872, when the act received royal assent, and the existing authorities were able to exercise their new powers from their first meeting after that date. The powers and responsibilities initially given to sanitary authorities in 1872 were relatively limited. They had to appoint a medical officer, but other powers were generally permissive rather than compulsory. Three years later the Public Health Act 1875 (38 & 39 Vict. c. 55) substantially broadened the scope of powers and expectations on sanitary authorities.

Urban sanitary districts were formed in any municipal borough governed under the Municipal Corporations Act 1835, in any improvement commissioners district formed by private act of Parliament, and in any local government district formed under the Public Health Act 1848 (11 & 12 Vict. c. 63) or Local Government Act 1858.

The existing governing body of the town (municipal corporation, improvement commissioners or local board of health) was designated as the urban sanitary authority.

When sanitary districts were formed there were approximately 225 boroughs, 575 local government districts and 50 improvement commissioners districts designated as urban sanitary districts. Over the next nineteen years the number changed: more urban sanitary districts were formed as towns adopted legislation forming local boards and as additional boroughs were incorporated; over the same period numerous urban sanitary districts were absorbed into expanding boroughs.

Rural sanitary districts were formed in all areas without a town government. They followed the boundaries of existing poor law unions, less the areas of urban sanitary districts. Any subsequent change in the area of the union also changed the sanitary district. At the time of abolition in 1894, there were 572 rural sanitary districts.