Wal-Mart CEO Lee Scott stunned the business world three years ago when he told a meeting of more than 1,000 suppliers and other partners near the company's Bentonville, Ark., offices: Environmental sustainability must become a central part of the company's formidable supply chain operations. The goals included everything from doubling fuel efficiency over the next decade to eliminating 30% of energy use in stores. And, the company would start taking such steps as evaluating suppliers on the environmental acceptability of their packaging and using those assessments to make buying decisions.

"Sustainability Is here to stay . . . a part of what all of us are going to be doing with our businesses from here on out," he said. With that widely reported speech, Wal-Mart became perhaps the most visible company to make the greening of its supply chain a priority. But it's hardly the only one. Driven by everything from the escalating price of gas to consumer demands for change, more companies are beginning to address the issue, in many cases, also incorporating new labor policies and other socially responsible practices in the mix. And their efforts include a wide variety of ambitious strategies—decreasing the number of truck deliveries by packing more product on each vehicle, designing products with reusable components and pressuring manufacturers to reduce carbon emissions, to name a few examples.

Still, companies face a host of challenges, some of which may be insurmountable. How, for example, do you ensure that your suppliers are following your new policies for, say, cutting carbon emissions or using only a proscribed set of chemicals? How do you persuade them to accommodate your policies if you don't have the reach of a Wal-Mart? And, just how do you measure results–and prove to your stakeholders you're doing more than talking big? "There are a great many difficulties associated with these efforts," says Nicole Darnall, assistant professor of environmental science and policy at George Mason University. "It can get very sticky."