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 is taking a closer look at each of the 18 top-flight clubs' summer transfer dealings so far. Next up in the series: VfL Wolfsburg.

How they fared in 2014/15…

It could hardly have been a better season for the Wolves, who not only finished runners-up in the Bundesliga and thus secured a return to the UEFA Champions League for the first time in five years, but they crowned it all off by winning the DFB Cup. Furthermore, they made it through to the quarter-finals of the Europa League and displayed an exciting, attractive brand of attacking football with Kevin De Bruyne excelling so much that he became the club's first player since Grafite (2009) to win Germany's Player of the Year award, with Dieter Hecking winning the coach's version.

Ins...

© gettyimages / David Rogers

Max Kruse (r.) (Borussia Mönchengladbach)

  • Scored 11 goals and provided ten assists last season
  • Gone into double figures in past three campaigns

What he offers: Reliable goalscorer who tracks back and gets involved in play; proven German international.

Carlos Ascues (Melgar)

  • Capped ten times by Peru, scored four goals
  • Previously played in Europe for SL Benfica and Panetolikos

What he offers: Defensive midfielder with glowing reputation, underlined at Copa America; sits in front of defence providing stability and security for back four.

Outs…

© gettyimages / Boris Streubel

Xizhe Zhang (r.) (Beijing Guoan)

  • Did not make an appearance for Wolfsburg, despite sitting on bench several times
  • Capped 11 times by China and voted Chinese FA Young Player of the Year in 2012
  • Scored 11 goals and set up 12 in 2013 season

 To-do list…

© gettyimages

Rather than buying any new players, Wolfsburg will be looking to ensure they resist any late offers for star players such as Kevin De Bruyne (l.), Ricardo Rodriguez and Bas Dost. All three have many suitors from top European clubs, but they also know what they have got in Wolfsburg. Embarking on a first Champions League campaign since winning the Bundesliga in 2009 will be a test of the depth of Dieter Hecking's squad and while Kruse and Ascues have been added, you still cannot help but feel that the numbers could be lacking when the fixtures start to come thick and fast for the Wolves.

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.