Some team such as West Ham United do not take their medicals very seriously, hence when they brought Andy Carroll injured for a massive fee in the summer, they were taken aback by the amount of time it took for him to get fit.
Others such as AC Milan do take their medicals very seriously. In 2009, they rejected the chance to sign then Porto left back Aly Cissokho for £13 million due to the state of his teeth.
Whether or not Milan made this up as an excuse to wriggle out of the deal we do not know. At the time the reasoning was very much laughable, although Cissokho's massive fall from grace since his Porto's days makes such a decision ingenious.
Today, Milan are up to their old tricks again as it has been revealed by Eurosport that Michael Essien has failed Milan's rigorous medical.
The reason for the failure is understood to be related to an old knee injury Essien picked up.
Throughout his career Essien has had a spate of injuries. They first began to accumulate in the 2008-09 season when he only competed in 11 Premier League games due to sustaining a anterior cruciate ligament injury, keeping him out for 6 months.
He suffered another injury in the 2009-10 season and he only played in 14 Premier League games.
The 2010-11 season he completed his first full season in two years, making 33 appearances in the Premier League; unfortunately, Essien's nightmare continued the following season after sustaining another anterior cruciate ligament injury and he only made 14 Premier League appearances.
Since then, Essien has been loaned out to Real Madrid and despite not suffering another injury during his spell at the Spanish club, Essien has failed to rediscover the spark that saw him regularly touted as the best midfield general during his peak.
Indeed, he only made 20 La Liga appearances for Real Madrid last season and this season for Chelsea he has only made 5 appearances.
In the past teams such as Real Madrid have spent millions on players such as Ronaldo (Brazilian) and Kaka, flouting the medical as a means of getting their big name player.
However, both players had shown on then recent form they were still among the very best in the world after long term injuries.
The last time Essien was seen as a world class player was in 2007 when he was a nominee for the Ballon D'or for the third year in succession. Prior to that, in 2006 Essien was voted as Africa's best player.
Since those heedy days Essien's injuries have robbed him of his greatest assets: his pace, power, stamina and ultimately his intensity.
At the age of 31, Essien will not regain those attributes back and his injuries are only likely to flare up under duress - something Milan have obviously taken into consideration.
Milan are understood to be undergoing further tests to rescue to deal - which would have been a free transfer, but as of now, the deal is very much under threat.