Pembrokeshire is a maritime county, bordered by the sea on three sides, by Ceredigion (Cardiganshire) to the northeast and by Carmarthenshire to the east.

The population (United Kingdom Census 2001) was 114,131. The administrative headquarters and historic county town is Haverfordwest and other settlements include Pembroke itself, Pembroke Dock, Milford Haven, Fishguard, Tenby, Saundersfoot, Narberth, Neyland and Newport. St David's, in the west of the county, is the United Kingdom's smallest city.

The highest point of the county is at Foel Cwmcerwyn (1759 ft/536 m).

The county boasts 170 miles (275 km) of coastline comprising important seabird breeding sites and numerous bays and sandy beaches. Almost all of the coast is included in the Pembrokeshire Coast National Park. A large estuary and natural harbour known as Milford Haven cuts deeply into the coast, being formed by the confluence of the Western Cleddau (which goes through Haverfordwest), the Eastern Cleddau and rivers Creswell and Carew. The estuary is bridged by Cleddau Bridge as part of the A477 between Neyland and Pembroke Dock : the next bridges upstream on the Cleddaus are at Haverfordwest and by Canaston Bridge.

Major bays include Newport Bay, Fishguard Bay and St Bride's Bay. There are many small islands off the coast of the county, the largest of which are Ramsey Island, Skomer Island and Caldey Island.

In the north of the county are the Preseli Mountains (Mynyddoedd Preseli), a wide stretch of high moorland with many prehistoric monuments and the source of the bluestones used in the construction of Stonehenge in England.

Elsewhere the county is relatively flat, most of the land being used for lowland farming of dairy cows, arable crops and oil seed rape.

The county was founded as a county palatine in 1138 with Gilbert de Clare as the first Earl of Pembroke. It has long been split between its mainly English-speaking south (known as "Little England beyond Wales") and its mainly Welsh-speaking north, along an imaginary line called the Landsker.

The Act of Union of 1536 divided the county into hundreds which followed with some modifications the lines of the ancient subdivision into cantrefs, which went back to before the Norman conquest. The 1536 hundreds were (clockwise from the north-east): Cilgerran or Kilgerran, Cemais or Kemes, Dewisland or Dewsland, Roose, Castlemartin, Narbeth and Dungleddy or Daugleddau.