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.

Over the next few weeks, bundesliga.com will be taking a closer look at each of the 18 top-flight clubs’ summer transfer dealings so far. Next up in the series it's the carnival club, 1. FSV Mainz 05...

How they fared in 2014/15...

It was an up-and-down campaign for Mainz last time out. The Karnevalsverein were one of four sides to go unbeaten for the first eight games of the season, as new coach Kasper Hjulmand's patient, passing philosophy appeared to be taking hold. A run of nine games without a win until the winter break, however, put the pressure on the Dane, and when the club slipped into the bottom two on Matchday 21, Swiss coach Martin Schmidt took over the reins. The 48-year-old returned to a high-pressing, aggressive approach and the threat of relegation was turned into a comfortable mid-table finish.

Ins...

Yoshinori Muto (r.) (Tokyo FC)

  • The 22-year-old Japanese striker is of the world's hottest properties,.
  • He netted a goal every 2.2 games in Japan's top-flight and has already made 13 international appearances.

What he offers: Eye for goal, ability to play up front or on the flank and pace to burn.

Fabian Frei (FC Basel 1893)

  • Cool, calm and collected Swiss holding midfielder, 26 years old.
  • UEFA Champions League experience: 23 games and four goals at Europe's top table.

What he offers: Hard-running style, unflustered passing game but equally strong in the tackle.

Leon Balogun (SV Darmstadt 98)

  • Nigerian international defender, 27.
  • Formed part of the Bundesliga 2's best defence in 2014/15 and managed to score four goals in the process.

What he offers: Powerful in the air, an excellent engine and dangerous from set-pieces

Outs...

Shinji Okazaki (r.) (Leicester City FC)

  • The Bundesliga's all-time record Japanese goalscorer, 29 years old.
  • Found the net 37 times in 128 German top-flight appearances.

Johannes Geis (FC Schalke 04)

  • At only 21 years old, one of Germany's most outstanding midfield prospects.
  • Featured in every Mainz game last season, picking up four assists and four goals.

Jonas Hofmann (Borussia Dortmund)

  • Versatile 23-year-old attacker loaned from BVB for 2014/15.
  • Managed a good return of three goals in 12 appearances in an injury-interrupted campaign.

To-do list...

Mainz coach Schmidt (l.) will not be lacking in the midfield department when FC Ingolstadt 04 come calling on Matchday 1. With no fewer than 13 players to cover the central and wide positions after the arrivals of Frei and Maximilian Beister, Schmidt is likely to have advised sporting director Christian Heidel to focus on reinforcing the front line and the defence. While Muto offers plenty of promise, he remains untried in the Bundesliga, and so an additional forward to compete with Florian Niederlechner for the lone striker's role would be a welcome addition. The retirement of Nikolce Noveski and injury to Pierre Bengtsson, meanwhile, have weakened the defence, where cover will be required.

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.