Special Areas of Conservation (SACs) are sites designated for their importance in Europe. While many are also SSSI's they may have slightly different boundaries or be designated for different features.
Together with Special Protection Areas (SPAs - Areas of particular importance to birds - though there are no SPAs within the National Park), SACs form a network of protected sites across Europe called Natura 2000 sites.
The SAC designation provides another level of protection for sites that are also SSSIs. The Welsh Assembly Government, through the Countryside Council for Wales is responsible to the European Union for ensuring that SACs remain in a condition favourable to the important habitats and species for which they have been designated.
See more information on the role and management of Natura 2000 sites on the Europa website.