Manchester United's bid to improve their defence has seen them spend a reported £50million to acquire right-back Aaron Wan-Bissaka from Crystal Palace.
The 21-year-old is yet to win a senior England cap but impressed in his breakthrough 2018-19 campaign, with United hoping he can tighten up a backline that conceded 54 Premier League goals last term.
Wan-Bissaka has not cost United as much as their record signing Paul Pogba, who joined from Juventus in a then-world record £89.3m deal in 2016, but he is one of the most expensive defenders of all time.
We profile the men at the back who have cost a fair whack.
VIRGIL VAN DIJK - Southampton to Liverpool, £75m
Liverpool tried unsuccessfully to buy Van Dijk in June 2017 and were accused of tapping him up by Southampton, but the defender's absence from several matchday squads thereafter meant a departure was never far away.
The Reds gave their fans a late Christmas gift in December of the same year, confirming the Netherlands international's arrival and he has looked worth every penny since making the switch to Anfield, winning the Professional Footballers' Association Players' Player of the Year last term and helping the club clinch their sixth Champions League title.
LUCAS HERNANDEZ - Atletico Madrid to Bayern Munich, £68m
Having featured as France's first-choice left-back in their triumphant 2018 World Cup campaign, it did not take long for other clubs to begin being linked with Lucas. Bayern were reported to have things tied up with the defender by the end of the year, though it was said they were trying to avoid paying his full release clause.
As it happened, Atletico received the €80m required to trigger Lucas' exit at the end of the 2018-19 season, meaning Bayern secured themselves a player who could cover two positions at the very top of the sport for the next decade.
AYMERIC LAPORTE - Athletic Bilbao to Manchester City, £57m
City had been put off by the fee Southampton wanted for Van Dijk but were more willing to part with a club-record £57m to sign left-sided French centre-back Laporte.
Though he remains without a senior cap for Les Bleus, Laporte has become a key figure under Guardiola and was City's best defender in 2018-19 as they won an unprecedented domestic treble.
BENJAMIN MENDY – Monaco to Manchester City, £51.75m
City broke the record for a defender when they signed Mendy from Monaco in July 2017, the left-back having quickly established himself as one of Europe's most exciting attacking full-backs as his side won the 2016-17 Ligue 1 title and reached that season's Champions League semi-finals.
His arrival was part of a spectacular overhaul of the City backline, which saw them spend close to £165m on three defenders and goalkeeper Ederson, but two serious knee injuries have badly hindered his time in Manchester so far.
DAVID LUIZ – Chelsea to Paris Saint-Germain, £50m
Paris Saint-Germain paid a whopping fee to sign Brazil international David Luiz from Chelsea in 2014.
He won Ligue 1 in both of his seasons at the Parc des Princes and made it three top-flight titles in a row after he surprisingly returned to Chelsea for £34m in August 2016, claiming his first Premier League winners' medal.
KYLE WALKER – Tottenham to Manchester City, £50m
When Walker joined Spurs from hometown club Sheffield United for £5m in 2009, few would have guessed he would one day become the most expensive English player in history.
Walker, who recently signed a new long-term deal at the Etihad Stadium, quickly established himself as a key component at the back following his July 2017 arrival and he has won two Premier League titles, two EFL Cups and an FA Cup.
AARON WAN-BISSAKA - Crystal Palace to Manchester United, £50m
Walker now has company as his country's most costly player following fellow right-back Wan-Bissaka's arrival on the other side of Manchester.
Wan-Bissaka fits the mould for Ole Gunnar Solskjaer, who wants young players to spearhead a revival at Old Trafford, and he made 129 tackles last term, more than any other defender across the top five European leagues.
JOHN STONES –��Everton to Manchester City, £47.5m
City have certainly shown a willingness to pay large amounts for English talent, with Stones joining the club from Everton in 2016.
The 24-year-old had a difficult first campaign as Guardiola's side under-performed. His displays have since improved considerably, although he largely spent the final weeks of last season among the substitutes.
EDER MILITAO – Porto to Real Madrid, £42m
Another reinforcement for 2019-20, Madrid caught many by surprise when it was announced earlier in March they had paid the release clause of Porto's Brazil international Militao.
The 21-year-old was only brought to Europe from Sao Paulo in 2018 and appears to represent something of a gamble by Los Blancos.
DAVINSON SANCHEZ – Ajax to Tottenham, £36.6m
Spurs waited several weeks to make their first signing of the close-season transfer window in 2017, but they did so in some style, smashing their club record to bring Sanchez to north London.
The former Atletico Nacional man reportedly caught Barcelona's attention during Ajax's run to the Europa League final in 2016-17 and has looked a solid addition to the Premier League since arriving.
(*Prices are as reported at the time of transfers being completed.)