More than 40 days remain in the 2015 summer transfer window, but ample business has been conducted across the world of football. The Premier League alone has enjoyed a lion's share of the headlines, seeing Manchester United, Chelsea and Liverpool strengthen.

Contrary to popular belief, however, club football extends well beyond England. Below are the three best summer transfers in European football thus far.

Honorable Mentions

Paulo Dybala, Palermo to Juventus, £23 million: Dybala was one of three players in Serie A last season who registered double figures for both goals (13) and assists (10).

Raheem Sterling, Liverpool to Manchester City, £49 million: The price tag is steep, but at just 20 years old, Sterling has already proven to be a dynamic weapon on the ball.

Douglas Costa, Shakhtar Donetsk to Bayern Munich, £21 million: Injuries to Arjen Robben and Franck Ribery forced Bayern Munich's hand. With Costa, the club boasts an additional attacking threat. In 14 Champions League matches last season, the 24-year-old completed 35 dribbles.

Bastian Schweinsteiger, Bayern Munich to Manchester United, £14.4 Million

For a mere £14.4 million—chump change by today's transfer market standards—Manchester United were able to acquire an engine for their midfield. Bastian Schweinsteiger has suffered with injuries in recent years, but the 30-year-old will bring authority and organization to a club who has long yearned for it.

Carlos Bacca, Sevilla to AC Milan, £21

In short, Carlos Bacca's decision to depart Sevilla for AC Milan was a peculiar one. After all, it was his former club that was ready to enjoy Champions League competition, not Milan, but the peculiarness to this decision is what makes it one of the summer's best. For just £21 million, AC Milan have acquired a proven goal scorer to ail their Serie A woes. Bacca finished his league campaign with 20 goals and six assists.

Jackson Martinez, FC Porto to Atletico Madrid, £25 Million

Speaking of proven goal scorers, Atletico Madrid lost one after Mario Mandzukic decided to join Juventus on a four-year deal. Never worried, Madrid quickly found a suitable replacement, signing Jackson Martinez from FC Porto. In the last three seasons, Martinez contributed 67 goals in just 90 appearances.

Note: Credit all statistics to officially licensed data from Opta Sports (via WhoScored).