Founded by a comedian with a penchant for swearing (early rallies were known as "Fuck You" days), refusing to describe itself as left or right, and above all eschewing the coalitions and alliances that are the hallmark of the modern Italian political system, in its early years the M5S certainly deserved the 'anti-establishment' label.
But co-founder Beppe Grillo recently said the party had entered its "adult phase", perhaps suggesting the rebelliousness of adolescence has been left behind.
Italy's political establishment comes in the form of a series of centre-right, centre-left, and occasionally mixed coalitions, which have ruled the country since the 1990s. That decade saw the existing political parties devastated by the collapse of Communist regimes around Europe and a major investigation into political corruption in Italy.
But the new parties that emerged – Forza Italia, the Democratic Party, and the Northern League being three of the main players – were far from free of corruption and scandal. And an electoral system that favoured cross-party alliances meant parties were regularly accused by supporters of selling out on key policies in order to get into government.
There are other reasons for voter dissatisfaction with the status quo, from a stagnant economy to mafia infiltration in politics, to a younger generation left disproportionately affected by unemployment and unstable work, after successive governments courted the elderly. The M5S started out with one message to voters: this is not the only option.
Current polling ahead of the March 4th election suggests the traditional centre-left and centre-right will both fall short of the majority needed to govern, while the M5S will likely emerge as the largest single party. But is it still an alternative to the country's political establishment, or just another form of the same?
Here's a look at just what has changed as the party's star has risen.
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A new leader
Luigi Di Maio's election as the party's new leader in September 2017 gave a clear sign of the movement's new direction.
Observers in Italy and further afield were quick to note the contrast between Grillo's unkempt appearance and profanity-filled rants and the sharp-suited, clean-shaven 31-year-old with a background in law and talent for public speaking.
Di Maio looks and sounds more like a politician than Grillo ever did. But he's far from a career politician, having fallen into the job apparently almost by accident after a brief stint as a student activist during a never-finished law degree. Critics point to his frequent mistakes in the Italian subjunctive tense as proof he is unprepared for the top job, though intellectual snobbery is only likely to strengthen support for Di Maio among the M5S base.
Since he took over, Grillo has quickly and openly distanced himself from the movement he founded, even formally separating his blog from the party blog.
However, there have been no signs of conflict between Grillo and the party. Professor James Newell, who teaches Italian politics at the UK's University of Salford, explains: "He has the image of a rabble rouser and probably concluded that taking a lower profile would help the party expand its support by attempting to cultivate an image as a credible governing actor.
"He was probably keen for the movement he created to do well even though it might involve some sacrifice of principles along the way."
Beppe Grillo (L) and the party's new leader Luigi Di Maio with the Five Star Movement's new logo. Photo: Andreas Solaro/AFP
The other candidates
Italian politics is not all about the leaders, though it might seem that way at times.
Part of the M5S's ideology is that politics should not be a career, so it imposes a two-term limit on its own representatives, after which they are expected to return to work in civil society. While the main parties are often accused of cronyism, the M5S excludes those who already hold public office from their list.
Di Maio said the list of candidates for the 2018 election was "the start of a new era, that of ordinary Italians in the institutions and government".
But this has not ensured that they are all are free of political scandal. One Senate candidate, Emanuele Dessi, was investigated by the party over claims he paid only €7 a month in rent on his apartment, but remains a candidate after saying he did not understand the rental contract he signed.
The party has power...
The paradox facing any 'anti-establishment' political movement is that if you succeed, you become the establishment you've railed against. Despite receiving 25 percent of the vote in the 2013 election, the M5S was kept out of government by its refusal to join an alliance, but has continued to gain support in regional votes since then.
The party seized control of several local councils in June 2016 elections, most importantly those in Rome and Turin, where it dethroned the governing Democratic Party (PD). But since then, the mayors have struggled.
In Rome, Virginia Raggi has been embroiled in corruption scandals which resulted in the party leadership stripping her of the power to make "important decisions", and locals have accused her of failing to deliver on a key campaign promise to tackle the city's rubbish crisis. Turin mayor Chiara Appendino got off to a more promising start, but in recent months the honeymoon period seems to have ended, with residents saying she is failing to adequately manage the city.
This may leave some voters disillusioned with the M5S's ability to deliver the changes it calls for, but others will likely still support them due to even deeper mistrust in the parties that governed their cities for years before the M5S took control.
Virginia Raggi pictured during a press conference. Photo: Filippo Monteforte/AFP
… but they want even more
The party is campaigning hard to reach the 40 percent threshold in the March 4th election, but they won't get there on protest votes alone.This means the party's politicians must adapt to gain wider appeal.
"The party has recognized that it has hitherto been a protest movement and now needs to present itself as a credible government-in-waiting if it is to advance," says Newell.
In order to do this, Di Maio travelled to London in January 2018 where he met with City investors, and told the Ansa newswire his message was "we're not populists".
This was just one of many meetings to reassure foreign diplomats, businessmen, and bankers, bringing the party close to the establishment it rails against. However, Italy's other main parties have much stronger links to big business, while the M5S has stuck to its policy of refusing large financial donations, promising to return any donations over €5,000.
In order to broaden its appeal, the M5S has dialled back some of its more radical proposals.
One of the biggest pivots has been the party's views on the EU and euro.
Grillo was fiercely eurosceptic and had long supported a referendum on euro membership, something which rattled markets and Italian business-owners. But in the months leading up to the 2018 election this was downgraded to a "plan B" and Grillo's replacement Di Maio has been more positive about the EU in general, recently saying he "would not contemplate" a referendum on the euro even as a last resort.
"We do not want a populist, extremist or anti-European Italy," he said in December.
In other areas, notably immigration, the party has been hard to pin down on policy.
At the core of the M5S is its belief in a new kind of democracy, one based on using the internet to let citizens to have their say directly, rather than decisions being made by politicians far removed from everyday life. Using an online platform, M5S members vote for election candidates as well as the party platform itself, and Di Maio recently launched an online portal for voters to choose which laws the party should abolish if it comes into power.
But former members have criticized the party leadership's tight control over the system. In 2017, a Genoa candidate was barred from the list despite topping the poll, when Grillo judged some of her positions to be "contrary to the principles of his movement". Writing on his blog later, Grillo did not expand on exactly what these positions, or the party principles supposedly violated, were, but instead called on members to "trust me" – prompting several defections from party members.
And in September, one of Di Maio's opponents in the leadership contest, Vincenzo Cicchetti, criticized the voting process, saying the Neapolitan was "imposed on us purely because he is young and telegenic".
What's more, only a small proportion of the total party membership are signed up to the voting platform, and even fewer actually vote. Even when they do, the party elite always get the last say – the M5S withdrew support for gay civil unions in parliament in 2016, despite 80 percent of voting members favouring the bill.
A cautious yes to party alliances
This is the biggest clue that the M5S is no longer the anti-establishment alternative.
Italy's new electoral law, which will be tested for the first time in March, favours coalitions, and M5S politicians protested against it on the grounds that their party was discriminated against.
The party has not joined any coalition ahead of the election, and Di Maio has repeatedly stated his belief they could reach the 40 percent threshold. But changes to its rules in January 2018 got rid of the stipulation which ruled out any kind of post-election alliance.
Di Maio says the M5S would not join a grand coalition, but more recently has said that in the event of an inconclusive event, he would invite other parties to support a M5S government on an agreed programme.
So, what happens if the M5S does get a place in Italy's next government?
For now, it is clinging on to its anti-establishment label through a combination of vagueness around its policies and the fact it has not yet made it into government, but if this changes after March, it would likely lose this label once and for all.
"Berlusconi promised radical change based on populist appeals and disappointed voters. The Northern League promised radical change on the basis of populist appeals and disappointed voters. The M5S is promising radical change on the basis of populist appeals. I think that when, in a few years' time, we look back on the current period in Italian political history we will conclude that Di Maio was destined to suffer the same fate as Silvio Berlusconi and [former Northern League leader] Umberto Bossi before him," predicts Newell.