Editor's Introduction: Green Technology Rules

As we mark the 40th anniversary of Earth Day, if you're trying to imagine a dividing line between the 20th and 21st centuries, you might find it in the alternative energy sector of green technology.

In this business, wind- and solar-based power look a bit quaint--despite their market leadership and potential for commercial scale.

The alternative energy sources we're talking about in this special series--hydrokinetics and ocean--are as much about technology as they are the human spirit...to innovate and overcome problems of the past.

Take hydrokinetics--a twist on hydo power, which avoids costly, damaging and unpopular dams, or a second generation of ocean-based energy that keeps much of the important gear on land.

Some of these technologies—algae based biofuel, for instance, have been under development for decades, which may qualify them for 20th century status, but turning points are very much of this age.

The amazing then about alternative energy is its diversity and flexibility. To can find it in so many sources and adapt it in so many ways.

In this series, you'll find stories about five cutting-edge technologies and the companies behind them, as well as a slideshow with a dozen-plus other companies attracting significant venture capital.

Of course, as much as we dream about future potential, it's also worth acknowledging past accomplishments. So much has changed since the first Earth Day in 1970, much of it thanks to advancements in technology affecting our everyday lives. Check out that story.

Which brings us full circle in a way. For all our good intentions about the environment and sustainability, they may only be as good as our technology.