In her resignation speech, Ms Sturgeon refused to back anyone as a successor, insisting there was a wealth of talent within the SNP.

John Swinney

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.

The 58-year-old 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.

Kate Forbes

The finance secretary has had a meteoric rise through the ranks of government. She was dropped into the job following the surprise resignation of Derek Mackay and was left to deliver the 2020 Scottish Budget with just hours notice.

Her performance since then has belied her relatively young age (32) and short parliamentary career.

She was first elected to the seat of Skye, Lochaber and Badenoch in 2016.

Few would be surprised if she took the next step up, to the highest office in Scotland, and she is rightly seen as one of the favourites.

It is not known whether she intends to stand, given that she is currently on maternity leave following the birth of her first child.

Angus Robertson

Angus Robertson has held leadership roles in the SNP before, having headed its Westminster group in a previous life as an MP.

That was a particularly prominent role following the 2015 election, when the SNP group swelled to 56 MPs and Mr Robertson frequently clashed with Prime Minister Theresa May.

However he lost his Moray seat in 2017, and was out of frontline politics for a spell before winning the Holyrood seat of Edinburgh Central in 2021.

He immediately took up a post in Ms Sturgeon's cabinet as constitution secretary - an important post given the more or less full-time conflict between the Scottish and UK governments.

Many believe the 53-year-old may see himself as a future first minister. The question may be whether that view is widespread among his colleagues.

Humza Yousaf

The health secretary is part of a newer generation of SNP figures, having become a Glasgow MSP in 2011.

He has held a number of senior posts in government, including as transport minister, Europe minister and justice secretary.

The 37-year-old is also widely seen as ambitious, and may fancy a shot at the top job.

That said, health secretary is a tough job at the best of times, and Mr Yousaf has been running the show at a historically difficult moment for the NHS.

Joanna Cherry

The lawyer has been a prominent MP in the SNP's Westminster group since she was first elected to Edinburgh South West in 2015.

She has been a critic of Ms Sturgeon's leadership on the issue of gender reform, and has built a following within sections of the party of a similar persuasion.

She opted to remain within the SNP while others left for Alex Salmond's new Alba Party.

Depending on the strength of feeling about the issue of gender reform within the party at the moment of the contest, Ms Cherry might see herself as a realistic contender.

However there would be questions about how the 56-year-old could control the party without being at Holyrood, with years to go until the next Scottish election.

She would need a strong deputy leader to take on the role of first minister in the meantime, because that post needs to be held by an MSP.

The SNP has had a leader sitting only at Westminster before - Nicola Sturgeon herself stood in as the Holyrood leader for Alex Salmond - but only when in opposition.

Mairi McAllan

The Clydesdale MSP is a fresh face at Holyrood, with the 30-year-old first being elected in 2021.

However she immediately stepped into government as environment minister, and has already been responsible for guiding foxhunting legislation through parliament.

A former solicitor, she served as a special advisor to Nicola Sturgeon before winning a seat of her own.

That means she has a solid grounding in how the Scottish government works - the question may be whether it is too early in her career to move into the very top job.

Neil Gray

Another recent arrival at Holyrood, Neil Gray was the MP for Airdrie but gave up the seat to win one at Holyrood in 2021.

The 36-year-old swiftly made his way into government as international development minister, and has taken the lead on Scotland's response to the refugee crisis resulting from the war in Ukraine.

He was seen as being close to Ms Sturgeon's leadership team, and could prove to be a continuity candidate.

Keith Brown

The justice secretary might be an outsider bet, but he has support within the SNP - evidenced by the fact he was elected the party's deputy leader.

The 61-year-old MSP for Clackmannanshire and Dunblane has been at Holyrood as long as the SNP has been in government, and has held a range of briefs from transport to the economy.

He declined to discuss any potential candidates in the wake of the resignation, saying that "today is about Nicola Sturgeon".