There are five constitencies covering Dyfed.
They are all county constituencies (CCs) of the House of Commons of the Parliament of the United Kingdom

Parliament of the United Kingdom

The Parliament of the United Kingdom of Great Britain and Northern Ireland is the supreme legislative body in the United Kingdom, British Crown dependencies and British overseas territories, located in London...

 (Westminster

Palace of Westminster

The Palace of Westminster, also known as the Houses of Parliament or Westminster Palace, is the meeting place of the two houses of the Parliament of the United Kingdom—the House of Lords and the House of Commons...

), and are used also, since the 1999 Welsh Assembly election

Welsh Assembly election, 1999

The first National Assembly for Wales elections were held on 6 May 1999. The overall turnout of voters was 46.3%. Although the Welsh Labour Party were the biggest party, they did not gain enough seats to form a majority government and instead entered into coalition with the Liberal Democrats...

, for elections to the National Assembly for Wales

National Assembly for Wales

The National Assembly for Wales is a devolved assembly with power to make legislation in Wales. The Assembly comprises 60 members, who are known as Assembly Members, or AMs...

.

Dyfed

Dyfed

Dyfed is a preserved county of Wales. It was created on 1 April 1974 under the terms of the Local Government Act 1972, and covered approximately the same geographic extent as the ancient Principality of Deheubarth, although excluding the Gower Peninsula and the area west of the River Tawe...

 is one of the eight preserved counties

Preserved counties of Wales

The preserved counties of Wales are the current areas used in Wales for the ceremonial purposes of Lieutenancy and Shrievalty. They are based on the counties created by the Local Government Act 1972 and used for local government and other purposes between 1974 and 1996.-Usage:The Local Government ...

 of Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

, consisting of the principal areas of Pembrokeshire

Pembrokeshire

Pembrokeshire is a county in the south west of Wales. It borders Carmarthenshire to the east and Ceredigion to the north east. The county town is Haverfordwest where Pembrokeshire County Council is headquartered....

, Carmarthenshire

Carmarthenshire

Carmarthenshire is a unitary authority in the south west of Wales and one of thirteen historic counties. It is the 3rd largest in Wales. Its three largest towns are Llanelli, Carmarthen and Ammanford...

 and Ceredigion

Ceredigion

Ceredigion is a county and former kingdom in mid-west Wales. As Cardiganshire , it was created in 1282, and was reconstituted as a county under that name in 1996, reverting to Ceredigion a day later...

.
These principal areas are also known as counties.

Boundary Commission for Wales proposals for new boundaries have been accepted, to become effective for the 2010 United Kingdom general election and the Welsh Assembly election, 2007.
The new boundaries will realign constituency boundaries with the boundaries of local government wards and reduce the electoral disparity between constituencies.

For Welsh Assembly elections, constituencies are grouped into additional member

Additional Member System

The Additional Member System is the term used in the United Kingdom for the mixed member proportional representation voting system used in Scotland, Wales and the London Assembly....

 electoral regions.
Boundary changes will affect the whole of Wales, and changes to constituency boundaries will mean, also, changes to regional boundaries.

Current boundaries

Constituency	Boundaries
1. Carmarthen East and Dinefwr CC (Westminster) Carmarthen East and Dinefwr (UK Parliament constituency) Carmathen East and Dinefwr is a constituency of the House of Commons of the Parliament of the United Kingdom. It elects one Member of Parliament by the first past the post system of election. It was created in 1997 mostly from the former seat of Carmarthen...
2. Carmarthen West and South Pembrokeshire CC (Westminster) Carmarthen West and South Pembrokeshire (UK Parliament constituency) Carmarthen West and South Pembrokeshire is a constituency of the House of Commons of the Parliament of the United Kingdom...
3. Ceredigion CC (Westminster) Ceredigion (UK Parliament constituency) Ceredigion, formerly Cardiganshire, is a constituency represented in the House of Commons of the Parliament of the United Kingdom. Created in 1536, the boundaries have remained remarkably unchanged for nearly five centuries...
4. Llanelli CC (Westminster) Llanelli (UK Parliament constituency) Llanelli is a constituency of the House of Commons of the Parliament of the United Kingdom. From 1918 to 1970 the official spelling of the constituency name was Llanelly...
5. Preseli Pembrokeshire CC (Westminster) Preseli Pembrokeshire (UK Parliament constituency) Preseli Pembrokeshire is a constituency of the House of Commons of the Parliament of the United Kingdom. It elects one Member of Parliament by the first past the post system of election....

New boundaries

Constituency	Boundaries
1. Carmarthen East and Dinefwr CC (Westminster) Carmarthen East and Dinefwr (UK Parliament constituency) Carmathen East and Dinefwr is a constituency of the House of Commons of the Parliament of the United Kingdom. It elects one Member of Parliament by the first past the post system of election. It was created in 1997 mostly from the former seat of Carmarthen...
2. Carmarthen West and South Pembrokeshire CC (Westminster) Carmarthen West and South Pembrokeshire (UK Parliament constituency) Carmarthen West and South Pembrokeshire is a constituency of the House of Commons of the Parliament of the United Kingdom...
3. Ceredigion CC (Westminster) Ceredigion (UK Parliament constituency) Ceredigion, formerly Cardiganshire, is a constituency represented in the House of Commons of the Parliament of the United Kingdom. Created in 1536, the boundaries have remained remarkably unchanged for nearly five centuries...
4. Llanelli CC (Westminster) Llanelli (UK Parliament constituency) Llanelli is a constituency of the House of Commons of the Parliament of the United Kingdom. From 1918 to 1970 the official spelling of the constituency name was Llanelly...
5. Preseli Pembrokeshire CC (Westminster) Preseli Pembrokeshire (UK Parliament constituency) Preseli Pembrokeshire is a constituency of the House of Commons of the Parliament of the United Kingdom. It elects one Member of Parliament by the first past the post system of election....

1999 to 2007

Constituency	Assembly region	Constituency boundaries
1. Carmarthen East and Dinefwr CC (Assembly)	Mid and West Wales
2. Carmarthen West and South Pembrokeshire CC (Assembly)
3. Ceredigion CC (Assembly)
4. Llanelli CC (Assembly)
5. Preseli Pembrokeshire CC (Assembly)

The Mid and West Wales region includes, also, two Powys constituencies and one which is partly a Gwynedd constituency and partly a Clwyd constituency.

From 2007

Constituency	Assembly region	Constituency boundaries
1. Carmarthen East and Dinefwr CC (Assembly)	Mid and West Wales
2. Carmarthen West and South Pembrokeshire CC (Assembly)
3. Ceredigion CC (Assembly)
4. Llanelli CC (Assembly)
5. Preseli Pembrokeshire CC (Assembly)

The Mid and West Wales region will include, also, two Powys constituencies and one Gwynedd constituency.