Hollywood bigwigs like Brad Pitt, Leonardo DiCaprio and Edward Norton rake in millions of dollars and millions of fans with every movie they make.
Off screen, though, they're taking on roles that some call the most important of their lives: speaking out about global warming and standing up for the environment.
"I think clearly the state of our environmental instability is the defining issue for our generation," Norton said.
Norton is just one of many celebrities with a vested interest in the environment. The passion might be in his blood -- the actor/director hails from a family of environmentalists.
His latest film, "The Painted Veil," is set in a lush region of China. It's a nod to his father, who founded China's first large-scale conservation movement.
"My engagement with these things is pretty much rooted in my father's engagement," Norton said.
Closer to home, Norton is continuing that family tradition with his own nonprofit, BP Solar Neighbors, which helps stars help others.
Every time a celebrity buys a solar power system for his or her home, another one is donated to a deserving family like Omar Mohammed's in South Central Los Angeles. Without BP Solar Neighbors, Mohammed wouldn't be able to afford his own solar power system.
"It was too costly -- way above our means to do it," he said, "so we are thankful for Ed Norton to give us the opportunity to model what it is to get sustainable energy in our community."
For concerned celebrities like Norton, interest in the environment is more than a passing fad. With commitment comes responsibility.
"It's a great position to be in, but it's a scary one," said actress Cameron Diaz. "As you want to be thoughtful and just don't want to go for the trend, you don't want people to jump onboard because it's popular. You want to be vested in it."
Practicing what they preach can be critical. Diaz drives a hybrid car, while DiCaprio produces television programs on the environment.
Brad Pitt is helping the environmental group Global Green rebuild New Orleans "green-style."
"We are excited about going green in New Orleans," Pitt said at a recent Global Green news conference.
As nonprofits battle to get their message out and compete for precious private donations, groups like Global Green admit that it helps to have celebrities on their side.
"To get a message out really demands and requires celebrity, the person who steps up and is willing to be a value leader," said Matt Peterson, CEO of Global Green.
Will Dana, editor at large for Rolling Stone magazine, agrees that star power helps sell the environmental story.
"Leonardo DiCaprio is not going to solve the problems, but if he gets people to understand it talking about it, that is good work on his part," Dana said.