When David Roberts was growing up near the oilfields of West Texas in the early 1960s, it never got dark. Back then, oilfields were lit 24/7 by the gas flares used to burn off natural gas, a byproduct of oil drilling. The flares released massive amounts of CO2, and over time, oil companies halted that harmful practice in the U.S. But gas flares remain the norm in the developing world—and today Roberts oversees a team at Marathon Oil that's trying to end the practice.

NEWSWEEK collaborated with three research partners to compile the rankings: KLD Research & Analytics, which tracks environmental, social and governance data on companies worldwide and served as lead partner; Trucost, which specializes in quantitative environmental performance measurement; and CorporateRegister.com, the world's largest online directory of social responsibility, sustainability and environmental reporting. Our goal was to assess each company's actual resource use and emissions and its policies and strategies, along with its reputation among its peers. The 500 companies included in the ranking are the largest U.S. companies as measured by revenue, market capitalization and number of employees. The companies are broken out into 15 sectors, based on the FTSE/Dow Jones Industry Classification Benchmark (ICB).

NEWSWEEK collaborated with three research partners to compile the rankings: KLD Research & Analytics, which tracks environmental, social and governance data on companies worldwide and served as lead partner; Trucost, which specializes in quantitative environmental performance measurement; and CorporateRegister.com, the world's largest online directory of social responsibility, sustainability and environmental reporting. Our goal was to assess each company's actual resource use and emissions and its policies and strategies, along with its reputation among its peers.