This study uses a multidisciplinary approach to elucidate the formation and evolution of a large coastal sand barrier complex in South Wales during the Holocene. Foraminifera, pollen, and geophysicale videncei s usedt o interprett he geometry,l ithology and stratigraphicarl elationships between deposits within the back-barrier area. Heavy mineral analysis and XRDA provide information on potential sediment sources. Geophysical surveys show that the western portion of the barrier at Pendine Sands rests on a ridge of Pleistocene glacigenic sediment. This study shows that the barrier formed during the early Holocene (ca. 8,000 to 7,000 years BP) in response to the drowning of the antecedent topography by rapidly rising relative sea-levels; lithostratigraphic and biostratigraphic evidence from cores recovered within the back-barrier area show that the high energy surf zone did not overstep the gravel ridge and rework the fossil cliffline behind the western portion of the barrier. Sediment reworked from glaci-fluvio deposits in Carmarthen Bay was supplied to the prograding dune system by strong westerly and southwesterly winds and longshore drift. Between ca. 6,200 and 5,700 years BP and between ca. 4,500 and 3,500 years BP the barrier underwent two phases of long-term stability. These periods of barrier progradation and stabilisation were punctuated by relatively short phases of erosion, instability and barrier breakdown. Periods of barrier stability were probably triggered by regressive phases in relative sea-level change, which promoted spit development, whereas the intervening instability and breakdown was probably caused by an increase in storm frequency. The response of this system to increased storm activity was primarily controlled by local topographic and sedimentological factors. The ridge beneath the western portion of the barrier prevented the total breakdown and or landward migration of the barrier dunes. Consequently, the back-barrier sediments deposited behind the barrier were preserved whereas the tidal inlet sequencese asto f the gravel ridge were reworked by wave action and tidal scour. Reclamation of the back-barrier area during the 17th and 19th centuries has had a significant effect on the configuration of the coastline at Pendine Sands. The construction of seawall defences stabilised the barrier dunes and promoted rapid accretion along the seaward side of the barrier dunes and at the distal end of the spit. The large expanse of sandflats which are exposed in Carmarthen Bay at low tide, and the frequency of strong westerly and southwesterly winds, were critical factors in the formation and development of the barrier dunes at Pendine Sands. The significance of antecedent topography indicates that the formation and evolution of this particular barrier should not be considered as typical of regional barrier development.