Sustainable investing hasn't just arrived; it's fast becoming a bedrock principle. We looked for companies that are taking up the sustainability challenge and found some that are addressing pressing environmental problems--too much trash, scarce resources and increasingly intractable weather woes.

Some of these firms have had sustainability at the core of their business long before it was a buzzword. Our six picks range from tiny, more-speculative firms whose success depends on their environmentally focused operations, to large, dependable firms that are giving at least part of their business a greener tint. We also look at three choices in the new but fast-growing field of green bonds.

Tackling Waste: Darling Ingredients

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The world's 7.5 billion people are contributing to an enormously untidy planet--not due just to trash and plastic but food waste, too. The world wastes about one-third of the food it produces--roughly 130 billion tons a year. That's about 200 to 250 pounds per person, according to the Food and Agriculture Organization of the United Nations.

Darling Ingredients (DAR, $27), which began its life as a Texas rendering company in the 19th century, collects food waste and various animal by-products and transforms it all into more useful things for customers in the food, animal feed and fuel industries, among others.

For example, Darling's Bakery Feeds division takes leftover bakery products--bread, dough, pasta, crackers, cereal,