William Nicholas Stone Courtney (16 December 1929 – 22 February 2011) was an Egyptian-born British actor. He was best known for his long-running role as Brigadier Lethbridge-Stewart in the BBC science fiction television series Doctor Who.

Nicholas Courtney was born 16 December 1929 in Cairo, Egypt, the son of Major Geoffrey William Courtney, a British army officer and diplomat, and Evelyn (née Stout). His paternal grandparents were the Oxford academic and journalist William Leonard Courtney and his first wife, Cordelia (née Place). His grandfather, W. L. Courtney, had been an amateur actor during his tenure at Oxford University and was involved in the early productions in the New Theatre by the Oxford University Dramatic Society. Nicholas' parents separated in 1931, when his mother left the household. He was raised by his father and stepmother, Anne (née Perrott), after the couple married in 1938.

Courtney was educated in France, Kenya and Egypt and attended a public school in Kent. On his maternal side he was descended from New Zealand politician John Cuff. He did his national service in the British Army, leaving after 18 months as a private, not wanting to pursue a military career. He moved to England to join London's Webber Douglas Academy of Dramatic Art. After two years of repertory theatre in Northampton, he became resident in London in 1961.

Courtney's first television work was in the 1957 series Escape. He made guest appearances in several cult television series, including The Avengers (1962, 1967), The Man in Room 17 (1965), The Champions (1968), Randall and Hopkirk (1969) and as a racing driver in Riviera Police (1965), and briefly appeared as a TV panel chairman in the film Take a Girl Like You (1970), hosting a debate between John Bird and John Fortune.

During William Hartnell's tenure as the First Doctor, director Douglas Camfield originally considered Courtney for the role of Richard the Lionheart in the serial The Crusade (1965), a role that ultimately went to Julian Glover. Camfield kept Courtney in mind for future roles, and later that year cast him in The Daleks' Master Plan, in which he played Space Security Agent Bret Vyon.

Camfield liked Courtney's performance, and when the director was assigned the 1968 serial The Web of Fear, he initially cast Courtney as Captain Knight. When David Langton gave up the role of Colonel Lethbridge-Stewart to work elsewhere, Camfield recast Captain Knight and gave the part to Courtney instead. The Lethbridge-Stewart character returned in the next season in The Invasion, promoted to the rank of brigadier, and in charge of the British contingent of UNIT. The organisation had been charged with protecting Earth from alien invasion. Years later, actor and writer Ian Marter (who played UNIT medical officer Harry Sullivan alongside Tom Baker) named a Russian military base used in The Invasion, but unnamed on screen, "Nykortny" in his novelisation of the story.

In this recurring role, he appeared semi-regularly in 101 episodes between 1970 and 1975. The character proved popular enough to return in 1983, first in Mawdryn Undead and in the official 20th anniversary special "The Five Doctors". Courtney made his final appearance in the 1989 serial Battlefield (although like many other former cast members, he reprised the role for the charity special Dimensions in Time). He appeared with Jean Marsh in both his first and last regular Doctor Who television appearances. Jean Marsh portrayed the character of Sara Kingdom in Courtney's first appearance, The Daleks' Master Plan, playing his character's sister. In Courtney's final appearance on the show, Marsh portrayed the villainous Morgaine in the Arthurian-inspired Battlefield. Marsh also appeared in the earlier story The Crusade for which Courtney had been considered. Courtney played Lethbridge-Stewart, either on television or in spin-off audio plays, alongside every subsequent Doctor up to and including Paul McGann, as well as substitute First Doctor Richard Hurndall. He did not act in the revived series, but appeared in archive footage. While he acted with Tenth Doctor actor David Tennant in the Big Finish audio dramas Sympathy for the Devil and UNIT: The Wasting, Tennant was playing a different character, Colonel Ross Brimmicombe-Wood, on both occasions.

Fifteen years after Dimensions in Time, Courtney returned as Lethbridge-Stewart (now, Sir Alistair), freshly returned from Peru, in Enemy of the Bane (2008), a two-part story in the Doctor Who spin-off The Sarah Jane Adventures, starring Elisabeth Sladen as Sarah Jane Smith. The Sarah Jane Adventures production team intended that Courtney would reappear in the following year's The Wedding of Sarah Jane Smith so that Lethbridge-Stewart would meet the Tenth Doctor, but Courtney was recovering from a stroke and unable to take part.