The natural world is the ultimate model, but it doesn't come with instructions on how humans can imitate it. William McDonough and Michael Braungart of McDonough Braungart Design Chemistry, Paul T. Anastas at the White House Office of Science and Technology Policy, and Julie B. Zimmerman at the U.S. EPA explain how the cradle-to-cradle vision sets a course for "What do I do" and how the 12 Principles of Green Engineering answer "How do I do it?".