Andre Schurrle hopes to feature often enough at Chelsea to earn a place in Germany's squad for the 2014 World Cup in Brazil.
Really happy
The move was long-mooted before becoming reality.
Schurrle added: "It was hard for me last summer that I didn't come here, but I wanted to be a professional, I wanted to do my best for Leverkusen. That's what I did.
"I'm really happy, I'm really looking forward to being a Chelsea player for the next years.
"I'm really happy now here and I really want to thank Leverkusen for making it happen."
And Schurrle, who played against Chelsea in the Blues' 2011/12 Champions League-winning campaign, is prepared for English football and optimistic of success.
"I know what's going on, what type of football there is to play," he said.
"There are so many good teams in the Premier League. There will always be Man United, Man City, Arsenal, Tottenham.
"Big clubs, big teams, good quality, but we have very good quality too."
