Back in the day, high schoolers looking at colleges were mainly concerned with three things: prestige, location, and whether the place had a rockin' social life. These days, however, applicants look for something more: a school with green credentials.

"Ten years ago, I don't remember any student asking me about green campuses," says Steven Roy Goodman, a college admissions strategist at TopColleges.com. "Now it's quite common for students to be keenly interested in how environmentally responsible colleges are."

Today, two-thirds of university applicants say that a school's environmental report card would influence whether they'd enroll, according to a survey by the Princeton Review, a test-prep company.

Elite schools competing for the best and brightest have taken the cue. Harvard University leads the pack in energy efficiency, Yale University revolutionized its food operations, and the University of California at Los Angeles revamped its waste management.

For Sierra's third annual ranking of planet-preserving colleges and universities, we e-mailed a lengthy questionnaire to sustainability experts at hundreds of schools. The survey covered eight categories: efficiency, energy, food, academics, purchasing, transportation, waste management, and administration. Schools could earn up to ten points in each category, and up to five bonus points if they had additional green initiatives. The final rankings, calibrated to create an out-of-100 rating system.

Rather than focus on one or two issues such as energy conservation and investment choices, we sought a comprehensive overview of each campus's sustainability efforts. -- Avital Binshtock and Michael Fox

Here's the Top 20. Click here for the full rankings of the top 135 schools.

1. University of Colorado - Boulder
2. University of Washington - Seattle
3. Middlebury College
4. University of Vermont
5. College of the Atlantic
6. Evergreen State College
7. University of California - Santa Cruz
8. University of California - Berkeley
9. University of California - Los Angeles
10. Oberlin College
11. Harvard University
12. University of New Hampshire
13. Arizona State University - Tempe
14. Yale University
15. University of Florida - Gainesville
16. Bates College
17. Willamette University
18. Warren Wilson College
19. Dickinson College
20. New York University