The deputy first minister is the most experienced option open to the SNP, given he has actually led the party before.
Mr Swinney became SNP leader in 2000 following the resignation of Alex Salmond but left the post in 2004 after a series of disappointing election results.
A former MP who has served at Holyrood since the dawn of the Scottish Parliament in 1999, Mr Swinney knows every inch of Holyrood and the government.
He would be the obvious choice to step in for Ms Sturgeon in the shorter term, given his role as her deputy, and he is widely seen as a safe pair of hands.
However there are major questions about whether he would actually want to take up the leadership again.