Are We Headed Toward a Green Bubble? Billions of dollars are pouring into clean-tech companies--with little results. Is this an intelligent investment or the beginning of another bubble?

The green bandwagon is rolling again.

At a time when most startups can't get funding and even government-backed small-business loans are scarce, there is seemingly endless enthusiasm for cleantech--that is, young companies that create energy out of renewable sources such as wind or sunlight, or make vehicles powered by something other than fossil fuel.

Cleantech is "the greatest economic opportunity of the 21st century, says Ira Ehrenpreis, board member of the National Venture Capital Association and general partner with Technology Partners, in Palo Alto, Calif. It is the only sector, he points out, that combines bipartisan political agreement, corporate and Wall Street support and a transformational issue--saving the earth's energy and water resources. "The green of the environment and the green of economic and financial returns go hand in hand, he says.