In archaeology, a broch /brɒx/ is an Iron Age drystone hollow-walled structure found in Scotland. Brochs belong to the classification "complex Atlantic roundhouse" devised by Scottish archaeologists in the 1980s.

Brochs are roundhouse buildings found throughout Atlantic Scotland. The word broch is derived from the Lowland Scots 'brough', meaning fort. In the mid-19th century, Scottish antiquaries called brochs 'burgs', after Old Norse borg, with the same meaning. Brochs are often referred to as dùns in the west, and they are the most spectacular of a complex class of buildings found in northern Scotland. There are approximately 571 candidate broch sites throughout the country, according to the Royal Commission on the Ancient and Historical Monuments of Scotland.

The origin of brochs is still subject to ongoing research. While most archaeologists believed 80 years ago that brochs were built by immigrants, there is now little doubt that the hollow-walled broch tower was an invention in what is now Scotland. The first brochs may have been built circa 300 BCE, and there is evidence to suggest that they were used primarily for defensive or offensive purposes.

The distribution of brochs is centred on northern Scotland, with the densest concentrations found in Caithness, Sutherland, and the Northern Isles. A few examples occur in the Scottish Borders and on the west coast of Dumfries and Galloway, and near Stirling. The original interpretation of brochs was that they were defensive structures, places of refuge for the community and their livestock. They were sometimes regarded as the work of Danes or Picts, and from the 1930s to the 1960s, archaeologists regarded them as castles where local landowners held sway over a subject population.

However, the castle theory fell from favour among Scottish archaeologists in the 1980s, due to a lack of supporting archaeological evidence. These archaeologists suggested defensibility was never a major concern in the siting of a broch, and argued that they may have been the "stately homes" of their time, objects of prestige and very visible demonstrations of superiority for important families. Once again, however, there is a lack of archaeological proof for this reconstruction, and the sheer number of brochs makes it problematic. The article concludes by stating that the purpose of brochs may have been a combination of defensive, offensive, and symbolic functions.

The word broch is derived from Lowland Scots 'brough', meaning (among other things) fort. In the mid-19th century Scottish antiquaries called brochs 'burgs', after Old Norse borg, with the same meaning. Place names in Scandinavian Scotland such as Burgawater and Burgan show that Old Norse borg is the older word used for these structures in the north. Brochs are often referred to as dùn in the west. Antiquarians began to use the spelling broch in the 1870s.

A precise definition for the word has proved elusive. Brochs are the most spectacular of a complex class of roundhouse buildings found throughout Atlantic Scotland. The Shetland Amenity Trust lists about 120 sites in Shetland as candidate brochs, while the Royal Commission on the Ancient and Historical Monuments of Scotland (RCAHMS) identifies a total of 571 candidate broch sites throughout the country. Researcher Euan MacKie, using a restricted definition, has proposed a much smaller total for Scotland of 104.

The origin of brochs is a subject of continuing research. Eighty years ago most archaeologists believed that brochs, usually regarded as the 'castles' of Iron Age chieftains, were built by immigrants who had been pushed northward after being displaced first by the intrusions of Belgic tribes into what is now southeast England at the end of the 2nd century BC and later by the Roman invasion of southern Britain beginning in AD 43. Yet there is now little doubt that the hollow-walled broch tower was an invention in what is now Scotland; even the kinds of pottery found inside them that most resembled south British styles were local hybrid forms.