National Assembly for Wales constituencies and electoral regions were first used for the 1999 elections. New boundaries came into use for the 2007 elections. The total numbers of constituencies and regions (40 constituencies and five regions) remained the same.
The constituencies of the National Assembly for Wales (or Welsh Assembly) (Welsh: Cynulliad Cenedlaethol Cymru) were created with the boundaries of the Welsh constituencies of the House of Commons of the Parliament of the United Kingdom (Westminster), as they were in 1999. The new boundaries were also used for the 2010 United Kingdom general election. Therefore, between the 2007 Assembly election and the 2010 United Kingdom general election, the two sets of constituencies, Assembly and Westminster, had differing boundaries.
Three constituency names, Conwy, Caernarfon, and Meirionydd Nant Conwy, have become historic, and the new boundaries define three constituencies with new names: Arfon, Dwyfor Meirionnydd, and Aberconwy. Generally, the new boundaries define each constituency taking account local government ward boundaries, and define constituencies close to equal in terms of the sizes of their electorates.
Unlike Westminster constituencies, Assembly constituencies are grouped into electoral regions, and an additional member system is used to elect four additional Assembly Members (AMs) (Welsh: Aelodau y Cynulliad) from each region, in addition to the AMs elected by the constituencies. The Electoral Regions boundaries were based upon the pre 1999 First Past the Post European Parliamentary Constituencies. At each general election of the Assembly, each elector has two votes, one constituency vote and one regional party-list vote. Each constituency elects one AM by the first past the post (single-member district plurality, SMDP) system, and the additional Assembly seats are filled from regional closed party lists, under the D'Hondt method, with constituency results being taken into account, to produce a degree of proportional representation for each region. Altogether, 60 AMs are elected from the 40 constituencies and five electoral regions, creating an Assembly of 40 constituency AMs and 20 additional AMs. Every constituent is represented by one constituency AM and four regional AMs.