There are four leading contenders for new reactor designs in the UK. These 'most eligible' designs for the UK (see Chapter 7) have in some cases been built overseas, and in others been licensed in one country or more (i.e. passed a detailed inspection of the reactor design by the national safety authority). Some reactor designs have also been 'pre-certified' by the US regulator, the Nuclear Regulatory Commission. In the past the US NRC did not certify reactors until a site was chosen, as the site's characteristics have some part to play in the final reactor design (e.g. in the provision of cooling water, seismic qualifications, etc.). Pre-certification is intended to reduce site development times by resolving major issues in the basic design in advance. None of the overseas licensing or construction work guarantees that a reactor will successfully complete the UK's licensing process, although issues that might have been stumbling blocks in a UK licensing process may already have been resolved elsewhere. The four designs are: EPR, AP600, Advanced BWR, and ACR-700.