That old corporate demon Wal-Mart is preaching renewable energy. DuPont is pioneering corn-based bioplastics. General Electric's NBC Universal unit turned off the lights in its studio smack in the middle of a Sunday Night Football broadcast last season and urged viewers to do the same in their homes. Most major American corporations are trying to board the environmental bandwagon, and practically any C.E.O. can expound on the threat of global climate change and the virtues of sustainable growth. But what does all this corporate green really mean?

Portfolio decided to take a closer look (see our coverage of the "green" trend). Concerned that many corporate environmental efforts may be degenerating into self-serving marketing stunts, we endeavored to separate the admirable (Whole Foods' purchase of enough wind-energy credits to offset 100 percent of its electricity use) from the laughable (the "green" holiday window display at Barneys department store last Christmas that recommended such must-have gifts as "a Prius in a pear tree").

Our team consulted dozens of government agencies, court records, and environmental watchdog groups. We also spoke extensively with representatives from the corporations themselves. We looked at firms in a variety of industries, not just the traditional smokestack polluters. This process, though informed by scientific data, contains a heavy dose of subjective judgment. Consequently, neither our Toxic 10 list of offenders nor our Green 11 roster of good guys should be seen as empirical rankings. These are merely companies that we think could be doing better, given their resources and position in their industries, or those that deserve to be commended. We hope our findings will inspire lively debate and, most of all, some serious thought about what corporations can do to really make a difference for the environment.