Most countries in Europe usually have three major titles which their clubs contest for viz: the national league, the national cup and the continental competition, and a treble is achieved when a club triumphs in all three fronts.
Winning a treble is arguably the highest honour a team can achieve as it shows near invincibility, dominating and conquering all that came before them in every tournament they participated in.
While some clubs might be on course to achieve this unique feat this season, there are still some obstacles in their path before they achieve immortality.
Winning a treble is an extremely rare feat such that only seven teams throughout history have managed to do it. In this piece, we shall be taking a look at the seven European clubs who have won a treble.
#1 Celtic (1966/1967)
The first club to ever win a treble in Europe were Celtic; the Scottish club managed by the legendary Jock Stein decimated all before them in the 1966/1967 season.
They won the Scottish Cup in April 1967, beating Aberdeen in the final, while also triumphing in the Scottish league, despite a tight race with bitter city rivals Rangers.
Despite the fact that The Bhoys were contesting in the European Cup for the first time ever, that did not stop them from going all the way to the final where they came up against the legendary and much more fancied Grande Inter managed by the iconic Helenio Herrera in Lisbon.
Celtic, however, defied expectations by defeating Inter Milan 2-1 to become the first British club to win the European Cup.
They also achieved the unique distinction of being the first club to win the title with all players from the same nation (since matched by Steaua Bucharest of Romania in 1987). In recognition of their efforts, the 1967 treble-winning team is affectionately known as 'The Lisbon Lions'.
What made Celtic's season even more spectacular was the fact that they not only won a treble but all five trophies which they contested as the Scottish League Cup and Glasgow Cup were also added, with the team scoring a whopping world record total of 196 goals in all competitions.
#2 Ajax (1971/1972)
While younger generations of football fans might only know Ajax as a club who only make up the numbers in the Champions League over the years, older fans know better than to think along those lines.
There was a time when Ajax were undisputedly the most dominant club in all of Europe, winning the European Cup on three consecutive occasions between 1970 and 1972.
Led by the mythical Johan Cruyff and ably supported by Johan Neeskens and Piet Keizer, Ajax reached the golden period of their history during this period, with their 'annus mirabilis' during the course of the 1971/1972 season when they won all three competitions they competed in.
The KNVB Beker was added to the kitty on May 11 1972, while the Amsterdam outfit also won the Eredivisie by a landslide before coming up against Inter Milan in the final of the European Cup at Rotterdam on the 31st of May.
The match was settled with a brace from Cruyff to give Ajax their third consecutive European Cup and complete the first and till date only treble in their illustrious history.
#3 PSV Eindhoven (1987/1988)
The next club after Ajax to win a treble were another Dutch side - PSV Eindhoven, who achieved what their Eredivisie rivals did 16 years earlier.
Coached by the legendary Guus Hiddink and with iconic players like Ronald Koeman and Edward Linskens, PSV overcame all obstacles before them to triumph on all fronts.
The Eredivisie was won with a nine-point gap, with PSV scoring a massive 117 goals in the process. The Dutch giants defeated Roda JC 3-2 in the final of the KNVB Beker to win the Cup for the fourth time.
In the European Cup, PSV saw off Galatasaray, Rapid Vienna, Bordeaux and Real Madrid to set up a final showdown with Benfica in Stuttgart.
Both teams could not be separated after 120 minutes as the match was tied in a goalless draw, with the lotteries of penalties being used to determine the winner.
The Dutch side triumphed 6-5 in the shootout, with Anton Janssen scoring the winning spot-kick to hand PSV their only European Cup victory till date.
#4 Manchester United (1998/1999)
Manchester United achieved immortality during the course of the 1998/1999 season as they became the first and till date the only English team to win a treble of the Premier League, the FA Cup and the Champions League.
They came into the campaign with limited expectations having finished the previous season trophyless, but the Red Devils defied expectations by going all the way in the three tournaments they contested.
The manifestation of United's never-say-die attitude was at its most glorious that season as the club overcame numerous difficult situations en route their spectacular success.
Arsenal engaged the Red Devils in an intense Premier League title race which was only settled on the last day by one point, while they saw off heavyweights, including Liverpool Chelsea and Arsenal in the FA Cup to set up a final clash with Newcastle which was won 2-0.
In the Champions League, they faced adversity in the semi-final as they found themselves 3-1 down on aggregate away to Juventus with less than 40 minutes remaining, but managed to scale through by winning 4-2 on the night.
They, however, saved their best for last, as they scored twice in injury-time to overturn a 1-0 deficit against Bayern Munch to complete their treble.
#5 Barcelona (2008/2009 2014/2015)
Incredibly, Barcelona have won the treble not once but on two occasions (the only team to have this distinction).
The first of this came in the 2008/2009 season as Pep Guardiola's tiki-taka philosophy was implemented to devastating effect on the field by Lionel Messi, Xavi Hernandez, Andres Iniesta among others and helped Barcelona decimate all before them.
Incredibly, Barcelona won all six trophies available in 2009 - adding the UEFA and the Spanish Super Cups as well as the Club World Cup to become the first team to achieve this feat, with many considering this Barcelona side to be the greatest club team in history.
Six years later, the Blaugrana - this time led by the devastating 'MSN' (Messi-Suarez-Neymar) won the second treble for the club, winning La Liga by two points, while also seeing off both Athletic Bilbao and Juventus by a scoreline of 3-1 to win the Copa Del Rey and Champions League respectively. Although they fell one short of matching the six trophies won in 2009 as Bilbao stopped them in the Spanish Super Cup.
#6 Inter Milan (2009/2010)
Just a year on from Barcelona's treble win in 2009, Inter Milan matched them and did so in some style as they put up one of the greatest defensive performances in history to triumph in the Champions League.
The Nerrazurri had earlier defeated AS Roma 1-0 in the final of the Coppa Italia, while also winning Serie A by a two-point margin over the same opponents.
On the continent, it was less straightforward, as the Italians were paired with defending champions Barcelona in the semifinal.
Back then, the Catalans were by far the most dominant club in the world and teams had to be at their A-game to defeat them.
Jose Mourinho knew that trying to attack Barcelona would end in a bad way for his side and as such, he conceded Barcelona's superiority, mounting a rearguard action that would e difficult to match again.
Over 180 minutes, Lucio and co kept Messi quiet as Barcelona probed in vain, being eliminated 3-2 on aggregate in front of their own fans.
In the final, Inter Milan came up against Bayern Munich at the Santiago Bernabeu and a Diego Milito brace was enough to hand them the trophy for the first time since 1965.
#7 Bayern Munich (2012/2013)
Bayern Munich lost the Champions League trophy in the cruellest way possible in 2012 - falling on penalties to Chelsea in a match which they had dominated from start to finish and to make matters worse, it was in front of their own fans.
They, however, made up for it in some style the very next season as they won the treble of Bundesliga, the DFB Pokal and the Champions League.
The Bavarians won the Bundesliga at a canter, finishing a massive 25 points ahead of Borussia Dortmund in second with a goal difference of +80, while they also saw Wolfsburg off 3-2 in the final of the DFP Pokal.
In the Champions League, they posted a 7-0 aggregate humiliation of Barcelona in the semifinal to set up a clash with Bundesliga rivals Borussia Dortmund in the final who they defeated 2-1 to hand them the first ever treble of their illustrious history.