23 SAS is one of the Territorial Army's special forces regiments. The regular SAS is known as 22, and originally there was one Special Air Service Reserve (SASR) regiment based in the south of Britain known as 21 SAS. Later, 23 SAS was created to encompass the north of the country, including cities such as Manchester and Newcastle-upon-Tyne.

In order to join an SASR regiment, individuals have to undergo a similar selection process to 22 SAS, albiet stretched over the period of more than a year. The Territorial Army SAS soldiers, although not part of the regular army, are still considered to be elite troops, and in times of war are said to be used in Long Range Reconnaissance behind enemy lines.

Due to the nature of special forces units, soldiers are targets for terrorist attack, and therefore the highest standards of secrecy are maintained.