Now that you've been exposed to the idea that saving the planet can also save you money, try on the concept of actually making money off environmentalism by investing in businesses in the "clean-tech" arena. Clean-tech companies supply products and services that support energy conservation, generation, and management, as well as water and wastewater treatment, eco-friendly agriculture, and other areas relating to the protection and sustainment of the natural environment. They also make money - sometimes a lot of it. The Cleantech Index, a group of stocks assembled by the Cleantech Group in Brighton, Michigan, returned nearly 20 percent of capital invested in the fi rst half of 2007, doing twice as well as the Standard & Poor 500, NASDAQ, and other broader indexes.

"This isn't socially responsible investing," stresses Ron Pernick, coauthor of The Clean Tech Revolution and cofounder of Clean Edge, a clean-tech consulting fi rm in Portland, Oregon. "This is technology driven." Pernick identifies six main factors that are propelling people's investment in cleantech businesses. Among them are higher and more-volatile prices of fossil fuels, falling costs of energy from alternative sources (such as the sun and the wind), and a wild card called China. Pernick expects that China will start becoming much greener - and when that happens, it will significantly affect the clean-tech industry.

One of the easiest ways to get on the clean-tech bandwagon is through an exchange-traded fund. For instance, First Trust Portfolios, a Lisle, Illinois, investmentmanagement firm, introduced in February the NASDAQ Clean Edge U.S. Liquid Series Index Fund, an exchange-traded fund that tracks the NASDAQ Clean Edge U.S. Index from Pernick's firm. The fund's biggest holding is First Solar, a solar-module maker in Phoenix that reported a net income of $44 million on sales of $72 million for the second quarter of 2007.

The small size and relative newness of companies like First Solar, which is a fairly typical fi rm in the world of clean-tech pureplay stocks, can make it tricky for a person to invest in shares of individual companies in the sector. Pernick says that clearly not all the clean-tech start-ups will prosper longterm, but some will likely do very well. "Last year, solar, wind, biofuels, and fuel cells were a $55 billion global industry," he says. "We see that expanding to greater than $225 billion in the next 10 years. That's why investor interest is being piqued."