Yesterday: retro-styled Converse tennies.

Today: urban trail shoes, flip-flops and Crocs.

Nothing stands still in the footwear industry -- and in the last 18 months, non-athletic shoe styles have gained popularity, said Mitch Kummetz, senior research analyst in Robert W. Baird & Co.'s Colorado office.

To a Wall Street type like Kummetz, that means one thing: new opportunities for certain companies and their investors.

"The companies that have done the best are brown shoe companies. Nike and Adidas don't do a wonderful job with this 'sport fusion' category, but Diesel and Steven Madden and Skechers do," Kummetz said.

Total U.S. footwear sales were $42 billion in 2005, up 9% from 2004, according to NPD Group, a market research firm. Sales of non-athletic footwear represented nearly 70% of that total, and were up 10.1%, compared with a rise of 4.5% for athletic footwear sales, according to the NPD data.

Kummetz says the category he calls "sport fusion" is one of the hottest in the non-athletic segment.

These shoes typically have an athletic silhouette but come in non-athletic colors like brown, gray -- even red and green -- and different materials like suede or Gore-Tex.

Sandals -- from sport sandals to flip-flops, both in a range of new styles -- also have been gaining popularity, with their sales rising 13.8% in the 12 months ended in June 2006, according to NPD.

"The flip-flop business has evolved from being a $1.99 purchase at Walgreens to the point where leather flip-flops are selling for $40 or $50," Kummetz said.

A number of companies are benefiting from these trends, including Deckers Outdoor Corp. (DECK, $57.16), Skechers U.S.A. Inc. (SKX, $32.53), Steven Madden Ltd. (SHOO, $36.06), Timberland Co. (TBL, $30.51) and Wolverine World Wide Inc. (WWW, $27.78).

Another such company, Ninot, Colo.-based Crocs Inc. (CROX, $43.68) is also benefiting after having single-handedly created a whole new category. Crocs, whose shoes have the same name, makes lightweight, clog-like shoes that are formed from closed-cell resin material and come in lavender, lime green and a wide variety of other colors.

Kummetz is familiar with all of these companies.

"I cover a slew of non-athletic footwear companies, but the one I'm most excited about these days is Wolverine," he said.

Wolverine, based in Rockford, Mich., is best-known for its Wolverine boots and Hush Puppies shoes. What's driving the company's performance now, though, is its Merrill brand.

Wolverine has grown Merrill to what Kummetz estimates is a $360 million business, from $20 million 10 years ago when the company acquired what was essentially a hiking boot brand.

"They've done a phenomenal job broadening it and really developing the rugged casual category to turn it into a casual footwear brand," he said.

Merrill brand sales have grown by 18% in each of the last two years, and the brand now represents nearly one-third of Wolverine's revenue, Kummetz said. At an analysts' event in New York in December, the company talked about its hopes for building Merrill into a $1 billion business, he said.

He thinks a big part of that growth will come from a new Merrill sports apparel line the company plans to launch next fall.

Wolverine has licensed the name Patagonia from the private company of the same name and will use it for a footwear line. The company will launch Patagonia footwear in spring. This line will likely be a $10 million-to-$12 million business, but Wolverine views it as an opportunity to produce $50 million to $100 million of annual sales over time, Kummetz said.

Wolverine has also done a good job of improving the profitability of its Hush Puppies business by giving it more contemporary styling and selling it at higher prices through higher-end stores, Kummetz said. The company also sells licensed footwear with the Caterpillar and Harley-Davidson brands.

The biggest risk Kummetz associates with this stock is the possibility its styles could fall out of favor, but Wolverine has a much more robust mix than many of its competitors, he said. There's also the possibility the company's business could be hurt by greater competition, particularly if more companies move into the popular non-athletic category, he said.

Kummetz has his highest "outperform" rating on Wolverine shares, and says they could go as high as $35 in the next 12 months.