The transfer window system has been a part of English football since the 2002/03 season, limiting every club to just two periods of buying and selling instead of the previous near all-year-round way of doing business.
The longer summer window is also particularly important in order to strengthen ahead of a new season, and a bad one can spell disaster.
From worst to best, here's a look at every summer window United have had in those 17 years.
Worst: Summer 2013
Ins: Guillermo Varela, Marouane Fellaini, Saidy Janko
Outs: Paul Scholes, Scott Wootton
Despite a lot of noise in the rumour mill, there was very little in the way of movement in summer 2013 when David Moyes and Ed Woodward replaced Sir Alex Ferguson and David Gill respectively. Marouane Fellaini unfairly bore the brunt of fan frustration, but the blame lay higher.
Ins: Diogo Dalot, Fred, Lee Grant
Outs: Michael Carrick, Joe Riley, Daley Blind
Summer 2018 was the beginning of the end for Jose Mourinho's time at Old Trafford, with £52m flop Fred the only major signing after the club hierarchy vetoed his other transfer targets. The club also let a proven and reliable performer in Daley Blind rejoin Ajax.
Ins: Ander Herrera, Luke Shaw, Marcos Rojo, Angel Di Maria, Daley Blind, Radamel Falcao (loan), Timoth Fosu-Mensah
Outs: Ryan Giggs, Alexander Buttner, Rio Ferdinand, Federico Macheda, Nemanja Vidic, Patrice Evra, Bebe, Shinji Kagawa, Danny Welbeck
On paper, United's summer haul when Louis van Gaal took over looked good. In reality, it was a huge disappointment, especially Angel Di Maria and Radamel Falcao. The squad was also gutted, losing the defensive leadership of Rio Ferdinand, Nemanja Vidic and Patrice Evra.
Ins: Memphis Depay, Matteo Darmian, Bastian Schweinsteiger, Morgan Schneiderlin, Sergio Romero, Anthony Martial, Regan Poole
Outs: Ben Amos, Tom Cleverley, Saidy Janko, Tom Thorpe, Nani, Robin van Persie, Reece James, Rafael, Angel Di Maria, Jonny Evans, Javier Hernandez, Anders Lindegaard
With Anthony Martial still struggling for consistency, backup goalkeeper Sergio Romero was probably the best signing of a prolific yet poor summer in 2015. United also lost money on Angel Di Maria just a year after his club-record arrival, and sold Nani and Rafael for a combined £7m.
Ins: Antonio Valencia, Michael Owen, Gabriel Obertan, Mame Biram Diouf
Outs: Carlos Tevez (loan ended), Cristiano Ronaldo, Lee Martin, Fraizer Campbell, Richard Eckersley, Manucho
United famously lost Cristiano Ronaldo to Real Madrid in summer 2009, as well as Carlos Tevez after a two-year loan, yet only spent sparingly. Antonio Valencia proved to be a long-term investment, but the club didn't do enough to stay ahead of Chelsea in the Premier League.
Ins: Victor Lindelof, Romelu Lukaku, Nemanja Matic, Zlatan Ibrahimovic
Outs: Zlatan Ibrahimovic, Josh Harrop, Wayne Rooney, Adnan Januzaj, Guillermo Varela
Zlatan Ibrahimovic was actually released in 2017, only to re-sign two months later. Clearly not fully fit, it was ultimately a mistake and he left for real within a few months. That year also saw the departure of all-time record goalscorer Wayne Rooney as part of the Romelu Lukaku deal.
Ins: Eric Bailly, Zlatan Ibrahimovic, Paul Pogba, Henrikh Mkhitaryan
Outs: Victor Valdes, Nick Powell, Paddy McNair, Donald Love, Tyler Blackett, Will Keane, James Weir
Jose Mourinho cleared out some young deadwood in summer 2016 and brought proven winner Ibrahimovic to Old Trafford. Henrikh Mkhitaryan ultimately flopped, but Paul Pogba could yet be the focal point of a new era if he stays, while the jury remains out on Eric Bailly.
Ins: Marnick Vermijl, Chris Smalling, Javier Hernandez, Bebe
Outs: Ben Foster, Tom Heaton, Zoran Tosic, David Gray, Rodrigo Possebon
United's transfer business in summer 2010 was unspectacular but effective. Chris Smalling completed an already agreed move from Fulham and Javier Hernandez proved to be a bargain find. Equally, no one of note left and United claimed a record-breaking 19th league title.
Ins: Phil Jones, Ashley Young, David de Gea
Outs: Edwin van der Sar, Paul Scholes, Owen Hargreaves, Wes Brown, John O'Shea, Gabriel Obertan
Summer 2011 marked the arrival of David de Gea at Old Trafford. He and fellow signings Phil Jones and Ashley Young still remain at the club, although only one of the trio has been a real success. That window also marked the first departure of Paul Scholes.
Ins: Owen Hargreaves, Anderson, Nani, Tomasz Kuszczak, Carlos Tevez (loan)
Outs: Kieran Richardson, Giuseppe Rossi, Alan Smith, Gabriel Heinze, Ole Gunnar Solskjaer
Owen Hargreaves, Anderson, Nani and Carlos Tevez could and perhaps should have been so much more at United in the long-term. That being said, each made a significant impact in their debut season in 2007/08 as the club won the Premier League and Champions League.
Ins: Fabio, Rafael, Dimitar Berbatov
Outs: Kieran Lee, Gerard Pique, Chris Eagles, Mikael Silvestre, Dong Fangzhuo, Louis Saha
Barring the departure of several fringe players, United retained the bulk of the Premier League and Champions League double-winning squad in summer 2008. Cristiano Ronaldo was persuaded to stay after flirting with Real Madrid, while Dimitar Berbatov arrived.
Ins: Edwin van der Sar, Park Ji-sung
Outs: Roy Carroll, Daniel Nardiello, Ricardo, Paul Tierney, Phil Neville, Kleberson
United didn't come close to challenging for the Premier League for a third straight season in 2005/06, but the summer that preceded the season still gave the club two players in Edwin van der Sar and Park Ji-sung, for just £6m combined, who were integral to future winning sides.
Ins: Michael Carrick
Outs: Quinton Fortune, David Bellion, Ruud van Nistelrooy, Paul McShane, Liam Miller
Michael Carrick was the only new face at United in summer 2006, with the £18m midfielder the final piece of the puzzle as the club went on to claim a first Premier League title in four years. It also didn't really matter that top scorer Ruud van Nistelrooy left to join Real Madrid.
Ins: Shinji Kagawa, Nick Powell, Robin van Persie, Alexander Buttner
Outs: Ritchie de Laet, Paul Pogba, Zeki Fryers, Tomasz Kuszczak, Michael Owen, Park Ji-sung, Dimitar Berbatov
Shinji Kagawa didn't take to English football and Nick Powell didn't deliver on his promise, but the arrival of Robin van Persie was what the summer of 2012 was all about. The Dutchman made all the difference as United claimed a 20th English title ahead of Manchester City.
Ins: Rio Ferdinand, Ricardo
Outs: Nick Culkin, Dwight Yorke
In what was the first-ever 'transfer window' after the new system was introduced, Rio Ferdinand cost United a British record £30m in summer 2002. He was worth every penny, though, winning 10 major trophies in 12 years. The club even recouped a little cash for a finished Dwight Yorke.
Ins: David Bellion, Eric Djemba-Djemba, Tim Howard, Kleberson, Cristiano Ronaldo
Outs: Lee Roche, Laurent Blanc, David May, David Beckham, Danny Webber, Juan Sebastian Veron
United were able to land an unknown teenager for just £12.24m in summer 2003, Cristiano Ronaldo. It wasn't an instant home run, but Ronaldo became the best player in the world at Old Trafford and steered the club to three Premier League titles and Champions League glory.
Best: Summer 2004
Ins: Alan Smith, Gabriel Heinze, Gerard Pique, Giuseppe Rossi, Wayne Rooney
Outs: Fabien Barthez, Danny Pugh, Mark Lynch, Nicky Butt, Luke Chadwick, Diego Forlan
United signed Wayne Rooney from Everton in summer 2004 and were rewarded with 252 goals over the next 13 years - he stuck around a lot longer than Ronaldo. Alan Smith and Gabriel Heinze both had moments in United shirts, even if they were very short-lived success stories.