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Manchester United are one of the biggest clubs in world football, and that status has naturally brought more than a few enemies along the way.
Some rivalries are born from geography, others from years of competing for trophies, while a few have been fuelled by unforgettable incidents and personalities.
For a club with such a long and successful history, there is no shortage of candidates when it comes to naming the Red Devils' biggest rivals.
The intensity has also changed over the years, with some historic feuds losing their edge while others have grown into defining fixtures of the modern game.
From Liverpool and Manchester City to Arsenal, Chelsea and Leeds United, FootballGroundGuide takes a look at United's five fiercest rivalries.
Liverpool
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Manchester United vs Liverpool sits at the top of the pile when it comes to sheer importance.
These are the two most successful and among the best-supported clubs in English football, with their rivalry stretching well beyond what happens on the pitch.
There is also a deep-rooted rivalry between the cities themselves, dating back long before modern football. The Manchester Ship Canal, constructed between 1887 and 1893, was built partly to allow Manchester traders to bypass Liverpool's port charges, and the longstanding Mancunian-Scouse antagonism has extended into almost every corner of popular culture.
On the football side, Sir Alex Ferguson famously declared that his greatest challenge was to knock Liverpool “right off their perch”.
The managers' battles added another layer, from Kevin Keegan's famous “love it” outburst in 1996 to Rafael Benitez's 2009 press conference and his infamous list of “facts”.
The two sides do not need a title at stake to make this fixture feel enormous.
Manchester City
Manchester City are United's most local rivals, and that alone guarantees a level of intensity that cannot really be manufactured.
The Manchester derby dates back to 1881, although City's rise to genuine English football superpower status has added an extra dimension to the fixture in the modern era.
This rivalry has meant different things in different eras. City's 1-0 win at Old Trafford in 1974 coincided with United's relegation, while the balance of power shifted decisively from the late 2000s onwards.
United's dramatic 4-3 victory in 2009, settled by Michael Owen in stoppage time, became an instant derby classic, before City's extraordinary 6-1 victory at Old Trafford in 2011 provided a brutal reminder that the hierarchy was changing.
Vincent Kompany's winner in April 2012 then put City above United on goal difference with only two games remaining, setting up one of the most dramatic title finishes in Premier League history.
The derby is no less fierce simply because the clubs are not always fighting for the same trophy.
Leeds United
There may be longer periods without this fixture than there once were, but Manchester United and Leeds United have one of the oldest and most culturally entrenched rivalries in English football.
The roots go back beyond the modern game, with the historical rivalry between Lancashire and Yorkshire providing a backdrop to a contest that began in football terms in 1906.
Leeds were one of England's major forces during the 1960s and 1970s, with the likes of Don Revie and Sir Matt Busby overseeing two hugely successful sides at Elland Road. The clubs repeatedly collided in the FA Cup, including semi-final meetings in 1965 and 1970, while United's 1977 victory over Leeds in another semi-final added another chapter to a fiercely contested relationship.
More recent memories include Eric Cantona's controversial move from Leeds to United in 1992 and United's 3-2 win at Elland Road in 2004 before Leeds were relegated.
It may not carry the same frequency or significance today, but the hostility has never disappeared.
Arsenal
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The United-Arsenal rivalry was once the defining contest of the Premier League.
It was not simply United against Arsenal, but Sir Alex Ferguson against Arsene Wenger, with Roy Keane and Patrick Vieira providing an equally explosive subplot.
The 1999 FA Cup semi-final replay remains one of the most iconic meetings. Peter Schmeichel saved Dennis Bergkamp's penalty in extra time before Ryan Giggs produced that unforgettable solo run and finish, keeping United's Treble dream alive.
Then came the infamous “Battle of the Buffet” in 2004, following United's victory over Arsenal that ended the Gunners' 49-match unbeaten Premier League run.
The rivalry produced tactical battles, tunnel confrontations and endless psychological warfare, while United's 8-2 demolition of Arsenal in 2011 remains one of the fixture's most remarkable scorelines.
Chelsea
Chelsea's rivalry with United is perhaps the clearest example of one built through sustained competition for honours rather than geography.
It became particularly fierce in the 1990s before Roman Abramovich's arrival transformed Chelsea into a genuine challenger to United's dominance.
Jose Mourinho's arrival in 2004 only intensified matters. Chelsea won the Premier League with 95 points in his first season and retained the title the following year, ending United's long period of domestic supremacy and creating another major managerial rivalry between Mourinho and Ferguson.
The teams then met in the 2007 FA Cup final and, a year later, the first all-English Champions League final. United won the famous Moscow final on penalties after a 1-1 draw, with Edwin van der Sar saving Nicolas Anelka's decisive spot-kick.
The rivalry has cooled compared with its peak years, but nearly two decades of important meetings have left a lasting mark.