Leonardo DiCaprio- has long been the most outspoken celebrity about environmental issues. He even encouraged other celebrities to arrive at the 2007 Oscars in hybrid vehicles. This year, Leo is releasing a documentary on global warming titled 'The 11th Hour'.

Alicia Silverstone -Famous vegetarian actress Alicia Silverstone has always been a huge supporter of animals and groups such as PETA. So, it should come as no surprise that Alicia has a completely eco-concious home. She spoke recently to InStyle magazine about the going green process.

Matt Damon -The talented Mr. Damon has been a good shepherd for the earth, narrating the documentary Running the Sahara and participating in the related awareness campaign about the water crisis in Africa. Damon also narrated the Journey to Planet Earth series on PBS.

Julia Roberts -This pretty woman was featured on the cover of Vanity Fair's green issue last year -- and for good reason: She's a spokesperson for Earth BioFuels and frequently talks up green issues on Oprah and other TV shows. Plus, the Erin Brockovich star will soon be playing another green activist, wildlife conservationist Joan Root, who was shot and killed in 2006.

Will Ferrell -He got guffaws during his blaze of glory on TBS's "Earth to America!" eco-comedy special, but this funnyman's commitment to the environment is no laughing matter. He's an active supporter of the Natural Resources Defense Council, and, though it may sound stranger than fiction, he'll soon be driving his electric car into the driveway of an eco-friendly home.

Edward Norton -The eco-commitment of this Hollywood heavyweight is no illusion. Norton used his star power to help launch BP's Solar Neighbors Program, which gets solar panels onto low-income homes in L.A., and he hosted an award-winning, four-part National Geographic TV special about the unnerving effects of global environmental problems.

George Clooney -This leading man not only earned an Oscar last year but also a prime spot on the cover of Vanity Fair's first green issue. In association with his award-winning role in Syriana, Clooney launched Oil Change, a campaign to say good night and good luck to America's dependence on oil. And to drive the point home, he's frequently seen out and about with his eco-friendly Tango electric car.

Cameron Diaz -This bombshell ain't trippin' (though that was the name of her eco-themed MTV show). The avid surfer drives a hybrid car, got trained to give Al Gore's climate presentation, and helped make the big announcement about Live Earth.