Invasions of the British Isles have occurred several times throughout their history. The British Isles have been subject to several waves of invasion and settlement since humans began inhabiting the region approximately 900,000 years ago during the Paleolithic. Notable invasions of the British Isles including the Roman conquest of Britain, Viking expansion, the Norman Conquest, the Anglo-Norman invasion of Ireland and the Glorious Revolution.

By around 12,000 BC, during the Mesolithic, Western Hunter Gatherers had started to repopulate Britain at the end of the Younger Dryas. A study by Brace et al. (2019) found evidence of a substantial replacement of this population ca. 4,000 BC, with the introduction of agriculture by Early European Farmers from continental Europe. According to the authors, "the transition to farming in Britain occurred with little introgression from resident foragers – either during initial colonization or throughout the Neolithic." This scenario has been questioned by British archaeologist Julian Thomas, who proposes a two-stage model of Neolithization, with the later second stage resulting in a more significant population transfer than the first.

Starting around 2400 BC, the Bell Beaker complex arrived in Britain, probably from the lower Rhine, an archaeological culture characterised by a new bell-shaped pottery style, and grave goods that included copper daggers and items associated with metallurgy and archery. The spread of this complex introduced high levels of Yamnaya-related ancestry from the Pontic-Caspian steppe into Britain, resulting in the "replacement of approximately 90% of Britain's gene pool within a few hundred years".

An earlier study by Cassidy et al. (2015) notes a "great wave of change" at the end of the Neolithic in Ireland, including the introduction of copper mines, metallurgy, tool and weapon production, and distinctive Food Vessel pottery. Their genetic study found that three Bronze Age individuals from Rathlin Island ca. 2026–1534 BC (calibrated) also carried significant levels of Yamnaya-related ancestry, in contrast to the preceding Neolithic population, suggesting a population turnover which the authors propose "invites the possibility of accompanying introduction of Indo-European, perhaps early Celtic, language."

Before the Romans came to Britain, and with them the advent of written records of the region, the majority of Britain was Celtic. How and when these peoples arrived in the British Isles is a matter of much conjecture; see Celtic settlement of Great Britain and Ireland for more details. The 11th-century Lebor Gabála Érenn describes successive invasions and settlements of Ireland by a variety of Celtic and pre-Celtic peoples; how much of it is based on historical fact is debated.

In 55 BC, Celtic Britain was invaded by the Romans under Julius Caesar. Caesar's two invasions did not conquer Britain but established it as a major trading partner of Rome.

A century later, a botched attempt to conquer Britain was made under the emperor Caligula. Caligula's uncle and successor, Claudius, was the first emperor to oversee a successful invasion. He used as an excuse the pleas for help that came from the Atrebates, Celtic allies of Rome, and landed an army near present-day Richborough. The initial landings were unopposed, and the Celts delayed in responding to the invasion. When, under their leaders Caratacus and Togodumnus, they did, they were too late and were defeated in several battles, most notably that of the River Medway.

Claudius arrived himself, bringing up to 38 war elephants with him. When the Celts were finally defeated and Caratacus forced to flee to Wales, Claudius returned to Rome.