Ynys Mon is a UK Parliamentary and Welsh Assembly constituency, covering the entire Island of Anglesey. The Parliamentary seat was first contested in 1536, long before the Welsh Assembly had even been thought about. The Assembly seat was first contested in 1999. Ynys Mon has a mixed political past, with Conservative, Labour and Plaid Cymru having returned strong results in Parliamentary and Assembly elections. The largest population centre is Holyhead.
Ynys Mon is coterminous with the Isle of Anglesey County Council local authority area. Following a reorganisation of authority in 2013, it is made up of the following electoral divisions: Caergybi, Ynys Gybi, Talybolion, Twrcelyn, Lligwy, Seiriol, Aethwy, Bro Rhosyr, Aberffraw, Llifon and Canolbarth Mon.