Green investing is growing up. Previously the province of a small number of investors who chased an even smaller number of companies, the market for environmental technology has expanded dramatically in recent years. And it has captured investors' wallet share along the way. Inflows into green funds totaled $766 million for the year ending May 31, according to Morningstar, compared with $37 million in net outflows from religious funds over same time period. (Morningstar tracks these two subcategories under the umbrella of socially responsible investing, or SRI, funds). "The interest has turned from 'maybe I'll dabble in this' to 'this is an asset class I should include in my portfolio,'" says Jerry Moskowitz, president of FTSE Americas.

Surging fuel prices have helped make green technology one of the biggest equity growth areas in the U.S., says John Quealy, a green tech analyst at Canaccord Adams, an independent financial services firm in Boston. New products are keeping pace. Mutual funds represent the largest share of socially and environmentally screened funds, with $171.7 billion in total net assets invested across 173 different funds, according to the Social Investment Forum's 2007 Report on Socially Responsible Investing Trends in the United States. Exchange-traded funds accounted for only 1% of the total assets of all socially and environmentally screened funds at the beginning of 2007, but their ranks are growing daily.

The source of investors' fascination is, of course, the need to find alternatives to oil and other fossil fuels in today's environment of scarcity and climate change. We've reached a point where environmental technologies have moved way beyond good wishes for Mother Earth, and are starting to make economic sense. Alternative energy has finally captured businesses' and investors' imaginations and the gold rush is on-and so are nascent fears of a bubble.

Oil prices of $140 or more per barrel highlight the scarcity-or at least, the fears of scarcity-of this hot commodity. Global demand for oil will only increase over the short term, even if record gas prices finally cause Americans to curb their consumption.

China and India are expected to more than double their energy use by 2030, according to the International Energy Agency. Increasing demand for fossil fuels pushes their prices up, which in turn spurs technological advances across all alternative energies. The world will continue developing better ways to power cars (the next iteration of the Toyota Prius, the ragingly popular gas sipper, will come with solar cells that help run its air conditioning), as well as alternatives to coal and other greenhouse gas emitters. Industry experts say that oil would have to drop back down to $50 per barrel for alternatives like solar, wind and geothermal energies to lose their economic viability.

Here's a closer a look at the opportunities, and some potential risks, in green investing.

Clean and Green

Investors considering environmental tech should start by defining their terms. For example, some use "clean tech" and "green tech" interchangeably, while others make a clear distinction. Jack Robinson falls into the latter group. The manager of the $410 million Winslow Green Growth Fund-whose three-, five- and 10-year performance has bested the Russell 2000 Growth Index-defines green companies as those involved in a bona fide, sustainable solution to global warming or other environmental ills; clean companies, in his parlance, are environmentally neutral. One clean company that Winslow holds in Green Growth is Bankrate, a North Palm Beach, Fla.-based online consumer banking and personal finance network. A green company he owns is Green Mountain Coffee Roasters, a Waterbury, Vt.-based purveyor of fair trade organic coffee that is carbon-neutral and donates 5% of its earnings to earth-friendly causes.

Investors must also decide for themselves what constitutes a green company. Quealy identifies four subcategories within the green sector: energy and power technology, which includes fuel cells; water technology; recycling technology; and bioresource technology, which includes ethanol. However, many so-called green companies don't draw 100% of their revenues from green activities. Purely returns-driven investors may not care to know the full scope of a company's endeavors, but those who see themselves as socially responsible will. "If you're large enough, you're going to be doing things some people don't like," says Peter Kinder, president of KLD Research & Analytics, a Boston-based social research and index firm.

Robinson won't hold companies that are at all "dirty." For example, he won't own BP, even though the British oil company is also developing alternative energies. Don Rogers, executive director of Virginia United Methodist Pensions, won't invest in conglomerates with more than 10% of their income from any combination of alcohol, firearms and gambling. Green investors also line up on different sides of the nuclear power divide, with some embracing the technology as an attractive alternative to fossil fuels and others shunning it as expensive, a cause of toxic waste and prone to accidents or terrorist attacks.