There have been three series, each consisting of three 90 minute long episodes, one each written by Steven Moffat, Mark Gatiss and Steve Thompson per series. Directors to date include Paul McGuigan, who has directed four episodes, and Euros Lyn, Toby Haynes, Jeremy Lovering, Colm McCarthy, and Nick Hurran, who have all directed only one each. The fourth series began with a special in January 2016.