Flag of the island of Anglesey, Wales Sticker

Anglesey (/ˈæŋɡəlsi/ ANG-gəl-see; Welsh: Ynys Môn [ˈənɨs ˈmoːn]) is an island off the north-west coast of Wales. It forms the bulk of the county known as the Isle of Anglesey, which also includes Holy Island (Ynys Gybi) and some islets and skerries. The county borders Gwynedd across the Menai Strait to the southeast, and is otherwise surrounded by the Irish Sea. Holyhead is the largest town, and the administrative centre is Llangefni. The Isle of Anglesey has an area of 275 square miles (712 km2) and had a population of 69,049 in 2022. After Holyhead, located on Holy Island, the largest settlements are Llangefni in the centre of Anglesey and Amlwch on the northern coast. The economy of the county is mostly based on agriculture, energy, and tourism, the latter especially on the coast. Holyhead is also a major ferry port for Dublin, Ireland. The county has the second-highest percentage of Welsh speakers in Wales, at 57.2%, and is considered a heartland of the language. The Isle of Anglesey is part of the preserved county of Gwynedd. The island of Anglesey, at 261 square miles (676 km2), is the largest in Wales and the Irish Sea and the seventh largest in Britain. The northern and eastern coasts of the island are rugged, and the southern and western coasts are generally gentler; the interior is gently undulating. In the north of the island is Llyn Alaw, a reservoir with an area of 1.4 square miles (4 km2). Holy Island, located off the east coast of Anglsey, has a similar landscape, with a rugged north and west coast and beaches to the east and south. The two larger islands are surrounded by smaller islands; several, including South Stack and Puffin Island, are home to seabird colonies. Large parts of the county's coastline have been designated an Area of Outstanding Natural Beauty. The county has many prehistoric monuments, such as Bryn Celli Ddu burial chamber. The medieval House of Aberffraw, which ruled the Kingdom of Gwynedd until 1283, originated on Anglesey and maintained courts on the island at (llysoedd) at Aberffraw and Rhosyr. After the Conquest of Wales by Edward I, Beaumaris Castle was constructed at the south-eastern corner of Anglesey; today it is part of the Castles and Town Walls of King Edward in Gwynedd World Heritage Site. During the nineteenth century the Menai Strait to the mainland was spanned by two bridges: the Menai Suspension Bridge, designed by Thomas Telford in 1826, and the Britannia Bridge, originally designed by Robert Stephenson in 1850.