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The highly-rated midfielder joined Celtic from Dundee United at the beginning of February 2015, along with his Tannadice team-mate, Gary Mackay-Steven.
Born in March 1992, Armstrong originally started his career at his hometown club, Inverness Caledonian Thistle, before moving to Dundee United in July 2009.
He made his debut for United in November 2010, coming on as a substitute in a home game against Kilmarnock. Over the next few seasons, he would go on to make 149 appearances for the club, scoring 21 goals.
This includes three goals against Celtic, the last of them coming in a 2-1 win for United at Tannadice in December 2014.
Armstrong would instantly bring his goal-scoring form to Celtic, bagging the Bhoys' third goal of the game on his debut against Partick Thistle at Firhill.
He would follow this with two goals on his first apperance at Celtic Park in the Europa League against Inter Milan.
Armstrong has been capped at Under-21 level for Scotland and was called into Gordon Strachan's squad for the Euro 2016 qualifiying ties against Georgia and Germany.
Away from football, the midfielder is studying for a law degree through the Open University