Daryl Hannah   For many years Hannah has lived simply using solar electricity and utilizing green building materials and practices in the construction of her home.   "I have been concerned about the environment for a long time, which is why I have been using biodiesel to fuel my car for the past six years. It's made from renewable materials such as vegetable oils or animal fats and has fewer emissions than petroleum-based diesel when burned. I have also converted my home to solar power." Leonardo DiCaprio   One of Hollywood's most committed environmentalists, he set up his own foundation and website (leonardodicaprio.com) to foster awareness of environmental issues. He also makes huge charity donations, has bought his own 104-acre Caribbean island, which he plans to develop into an eco-friendly holiday resort with self-sustaining energy sources and is currently producing and narrating a feature-length documentary film called 11th Hour about global warming.   "Global warming is not only the number one environmental challenge we face today, but one of the most important issues facing all humanity," says the 32-year-old actor, who has already made two shorter films, Global Warning and Water Planet. "We all have to do our part to raise awareness." Woody Harrelson   An active environmentalist, Harrelson's website (voiceyourself.com) shares information about fuel alternatives, sustainable material (i.e. hemp, organic cotton and bamboo) as well as natural cleaners.   "Since I was a kid I felt this need to play a role in helping Mother Earth. At the age of 12, I wrote a 50-page report (that wasn't supposed to be more than five pages) on threatened wildlife and realized clearly that my Mother was being continually and forcibly raped by giant corporations intent on profit. I see the ongoing destruction of all that is natural and want to make a difference." Cameron Diaz   She produced and starred in the TV series Trippin', in which she and some celebrity friends travelled the globe to get a closer look at various eco-systems. She was also one of first celebs to drive a hybrid car.   "Too many of us are so environmentally unconscious," says the 34-year-old actress. "And, while Mother Nature will eventually recover down the line, human beings won't be around to see the rejuvenation. I've met so many people across the globe, not just in our own backyards and got to see first hand how we can help it." Brad Pitt   Pitt recently narrated a six-part series on PBS about eco-friendly architecture. The star has also donated money to Future Forests, an organization that plants trees and he recently traded in his gas-run vehicle for an electric one.   "We have the technology for electric cars, and we have other power sources. Yes, it would be expensive to make the switch over, but we could, and we would have complete autonomy from our need for oil." - Max Hardy