It is often the final hurdle, but some big names have seen major transfers scuppered by medicals that didn't go to plan.
Once fees are agreed and personal terms are negotiated, clubs like to go through the formalities and check their incoming player has a full bill of health. But when a concern, no matter how small, is flagged it can often mean the deal is off - as has been the case with some of the Premier League's leading lights.
With transfer fees going through the roof and recruitment seemingly more important than ever, some teams are simply not willing to take the chance, even if the player does have plenty of promise or quality.
Manchester United, Liverpool and Chelsea have all walked away from moves after discovering concerns whilst going through the standard procedures of a medical. Some of those stories have had happy endings, but others haven't with players missing out on moves that could've made a career.
Ruud van Nistelrooy
A case of not now, but later. Van Nistelrooy was scoring goals for fun in the Eredivisie with PSV - earning individual accolades along the way. That caught the attention of Sir Alex Ferguson, who wanted to bring him to Old Trafford in the summer of 2000.
A club-record move was in the pipeline with a press conference date in the diary, but it would eventually be used to announce that the deal was off. A medical had flagged issues with Van Nistelrooy's knee and he returned to Eindhoven, where he would rupture his ACL, leaving him sidelined for a year.
Once he recovered United came back in and completed a switch that, in the early 2000s, represented a huge investment at £19m. The Dutchman played down his fee and quickly set about repaying it - eventually scoring 150 times during his five years in the Premier League.
Radamel Falcao
The Colombian headed to England with an excellent reputation and Chelsea opted to take a put on him despite an underwhelming spell at United the season before. Falcao would struggle to recapture the excellent form he'd shown on the continent and Monaco, his parent club, weighed up whether to bring him back.
The striker headed to the south of France for a quick health check, only for Monaco to find out he was injured. The club's then vice-president Vadim Vasilyev confirmed that the Ligue 1 outfit opted to leave him in west London.
He said: “The original idea was that he would return to Monaco to play the second half of the season. He came over and had a medical. Unfortunately, he was injured. So we took the decision that it would be better for him to stay with Chelsea.
Loic Remy
QPR were resigned to losing their French forward and Liverpool looked to have won the race before a heart issue surfaced in his medical, which saw the Reds pull out of the deal, thus opening the door for Chelsea to swoop in.
Remy had been training with Liverpool under Brendan Rodgers ahead of a £8.5m move before they found some heart concerns. The player has told the Daily Star: "They asked for a specialist for the heart and apparently the guy said they did not 100 per cent agree if I could keep going or had to stop football."
Harry Redknapp, then in charge at Loftus Road, was left baffled by what the Merseyside outfit discovered. "He had a stringent one (medical) here, one at Marseille and at Newcastle and with France before the World Cup. You couldn't meet a fitter lad," he said. Remy would go on to win a Premier League winners medal in his first year with the Blues.
Nabil Fekir
In the summer of 2018 Liverpool were looking to make a statement with the £53m signing of Lyon star Nabil Fekir. The Reds, who had just lost the Champions League final, were looking to close on domestic rivals Manchester City and wanted to bolster their attacking ranks.
Fekir had completed the formalities, even completing a announcement video with LFC TV, before the Reds pulled out late on amid concerns over the player's knee. The Frenchman had suffered a ruptured cruciate ligament during the 2015/16 campaign - and clearly issues remained.
The player himself, who moved to Real Betis for £12m just 12 months after Liverpool's mammoth offer, denies that there were any issues during his medical. He told L’Equipe: “Lots of lies were told and they affected me. What was said is not the truth. My knee is good – I had a quick medical check at Clairefontaine [France’s national training base] and my knee wasn’t even mentioned."
Marko Arnautovic
Chelsea were looking to tie up a deal for the youngster, who had excelled at FC Twente, in 2009, but a medical would reveal a foot fracture that saw the Blues walk away from a £12m move. Arnautovic's agent, Robert Groener, said: "Chelsea are now doing nothing."
The Austrian would instead move to Inter Milan on loan before heading to the Bundesliga with Werder Bremen. Arnautovic would find his way to the Premier League in 2013 when he signed for Stoke. He spent four years with the Potters before he pushed for a move to West Ham.
