Cologne - While the Bundesliga's many stars continue to build up their fitness levels ahead of the new season, clubs up and down the country are working tirelessly behind the scenes to strengthen their playing staff in time for the big kick-off in mid August.

bundesliga.com has been taking a closer look at each of the 18 top-flight clubs' summer transfer dealings so far, and we end our series with the champions, FC Bayern München.

How they fared in 2014/15…

Last season went according to plan for FC Bayern, who remained at the top of the Bundesliga from Matchday 4 right through to the end of the campaign, dominating almost at will with their possession-based football. It earned them a third straight Bundesliga title, and the obvious aim for this season is to become the first side to win four straight league titles in Germany. To ensure that they can go even further in other competitions, notably the UEFA Champions League, a few additions have been made to the playing staff.

Ins…

Arturo Vidal (r.) (Juventus)

  • Made 117 Bundesliga appearances, scoring 15 goals for Bayer 04 Leverkusen
  • Averages a goal every three Champions League games

What he offers: Became even more versatile at Juve, a midfielder who can do it all – strong in the tackle with a lethal finish, also from long-range. Tireless, energetic and a true leader.

Douglas Costa (FC Shakhtar Donetsk)

  • Five-time champion of Ukraine with Donetsk
  • Champions League ever-present last term, scoring one and setting up one

What he offers: Just what Pep Guardiola was lacking, an additional fast player down either wing, giving more width to Bayern's game.

Joshua Kimmich (VfB Stuttgart)

  • Won UEFA European Under-19 Championship title with Germany in 2014
  • Yet to appear in the Bundesliga

What he offers: Extremely confident and assured midfielder, despite tender age; likes to take initiative and have possession – perfect for Pep.

Outs...

Bastian Schweinsteiger (r.) (Manchester United FC)

  • Won eight Bundesliga titles for Bayern – a club record
  • Made 500 appearances for the Bavarians
  • Spent 17 years with record German champions

Claudio Pizarro (free agent)

  • Bundesliga's most prolific foreign player (176 goals)
  • Six-time Bundesliga champion

To-do list…

With the arrival of Vidal from last season's beaten Champions League finalists Juventus, Guardiola appears to have placed the final piece in his jigsaw. The priority has been on strengthening the squad in terms of numbers and Guardiola has succeeded by bringing in alternatives in key positions, such as Vidal and Kimmich, as well as a prolific one-on-one player in Douglas Costa. He will also be hoping Franck Ribery and Arjen Robben remain free of injury to ensure a keen battle for places which will keep everybody on their toes, and enough matches to keep them all happy.

Editor's note: Not all summer transfers will be discussed here. The players featured in this article made at least ten league appearances for their respective clubs last season, chipped in with numerous goals and assists or made other significant contributions to their team. For an overview of the 2015/16 summer transfers, click here.