 Three series , each consisting of three episodes , have been produced . The first series was initially broadcast in July and August 2010 on the BBC , later premiering on Public Broadcasting Service ( PBS ) in the United States in October 2010 . A second series of three episodes was first broadcast in the UK in January 2012 , and then in the U.S. during May 2012 . The third series premiered in the UK on 1 January 2014 and in the US on 19 January 2014 . The series has been sold to over 200 territories . A special episode premiered on 1 January 2016 , on BBC One and PBS , marking the first time the series has aired on the same day in the UK and U.S. 
