LONDON , England ( CNN ) -- For Europe 's biggest leagues the football season has ended , the boring business of kicking a ball about is over , and now the real fun can begin as the transfer window opens . Cristiano Ronaldo has left Manchester United to join Real Madrid for a world record fee of $ 130 million . What is the transfer window ? The transfer window is a set period of time when football clubs are allowed to buy new players . Although players ca n't be bought outside the transfer window it does n't stop clubs negotiating transfers before the window opens , with players signing on the dotted line once it does . There are two transfer windows per year , one pre-season , and one mid- season , which is open throughout January . The English Premier League pre-season transfer window opened at midnight on the last day of the season ( May 24 ) and will close on August 31 , while in the other big European football nations ( including France , Germany Italy and Spain ) the transfer window opens on July 1 and closes at the end of August . Other countries operate pre-season transfer windows to fit the calendar of their own football seasons . See our choice of the top 20 transfer targets » . Why does it matter ? The transfer window can see some of football 's biggest stars switch club and it is a period that can determine how a team will fare during the season ahead . It is a chance for coaches to rebuild and reinforce their squads , and a time when clubs can see their key players depart for greener pastures and bigger wage packets . When a club is active in the transfer window , making big name buys , it can be taken as a statement of intent for the following season . Just watch Spanish club Real Madrid bring in star players in an attempt to make up the distance between them and arch-rivals Barcelona . The global economic downturn means many clubs may not spend as much money this summer as they have in recent years , or that they may be more willing to trade or part exchange players if they are unable to meet a player 's price tag . The final day of the transfer window , known as deadline day , often brings a frenzy of activity as teams try to finalize deals before the window slams shut , drawing the curtains until January 1 , when the transfer fun begins again . Keep up with the latest transfer gossip with our transfer blog . Movers and shakers . This summer 's transfer window is likely to be dominated by one club -- Real Madrid . Their newly elected club president , billionaire construction magnate Florentino Perez , previously headed the club between 2000 and 2006 , when his policy was to bring football 's biggest names to Real Madrid . The `` Galacticos '' brought to Real by Perez included the likes of Luís Figo , Zinedine Zidane , the Brazilian Ronaldo and David Beckham . Now Perez is back in charge and seems intent on reviving his `` Galacticos '' policy , rsigning former World Player of the Year Kaka before smashing the world transfer record to spay $ 130 million for Manchester United 's Cristiano Ronaldo . Madrid have also been linked with Bayern Munich 's French winger Frank Ribery , Liverpool midfielder Xabi Alonso and Valencia winger David Silva . But Real wo n't have it all their own way , facing competition from the likes of Chelsea . New manager Carlo Ancelotti is bound to have brought a shopping list with him and the club 's Russian billionaire owner Roman Abramovich will want to show that he still has the financial muscle to compete on the transfer money-go-round . Then there is Manchester City , funded by the vast oil wealth of the Abu Dhabi United Group . The owners have so far struggled to attract big names to the club and will be keen to show that City are genuine Champions League material . At the other end of the spectrum Spanish club Valencia are in all kinds of financial difficulty and may be forced to sell their biggest assets , with David Silva and striker David Villa the most prized of all .
The transfer window is the only time football teams can buy new players . Real Madrid and Barcelona are likely to be the biggest - selling teams . Arsenal and Newcastle United will be looking to sell some of their stars .