The definition of summer seems to get looser with every passing season. In 2010, summer officially begins on June 21. Yet, according to Hollywood -- and us! -- summer begins this year on May 7. Why May 7? Because that's the day when 'Iron Man 2' is released and, without question, that's a summer action movie. How can you tell if a film is truly a summer action movie? Well, usually, the film will star the biggest names in Hollywood and, yes, things around them are exploding. Join us, if you will, while we take a look at the biggest action films to look forward to this summer.