Comfy, stretchy pants — the ones with the forgiving waistband and breathable fabric — have always had their place: weekends on the couch and sometimes at the gym.

But companies are now trying to turn this icon of casual Americana on its head by taking it mainstream, and even into the office.

Betabrand, the Internet clothing company and maker of “executive” hoodies, said that its dress yoga pants that first shipped in February quickly became its top-selling product. (Maybe you’ve seen those ads of a woman doing a dancer pose in her cubicle.) And upstart Sweat Tailor, which sells $98 tailored sweatpants for men, told Fortune that its three-day Kickstarter soft launch raised $14,000. The company, whose campaign officially starts Tuesday, had expected to rake in at most $3,000. “What we really focused on and wanted to do was create a pant that has the comfort of sweatpants with the fit of jeans or chinos,” says co-founder Aaron Hoffman. “There’s obviously a market for that.”

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The idea of merging comfort and style isn’t all that novel. Pajama jeans certainly have their place in fashion history, as do Kanye West’s leather jogging pants. What’s new is the booming “athleisure” trend that these companies are entering into.

As Americans attempt to be more active — or at least look as if they are — sportswear is going mainstream. “The yoga pants and outdoor jackets originally designed for more technical athletic pursuits are being worn on a daily basis,” says David McGoldrick, a research associate for Euromonitor. “Now they have crossover appeal.” The athleisure trend is not just for adults. The all-important teenage segment is on to it. In its “Taking Stock with Teens – Spring 2014” survey, Piper Jaffray found that 13% of teenagers listed a denim company as their favorite brand, down from 20% in 2010. Meanwhile, the percentage of teenagers who prefer an athletic brand most surged from 2% to 14%.

Sportswear sales growth has outpaced overall apparel sales, according to Euromonitor. In 2010, apparel sales increased 2% while sportswear sales surged 6.7%, eclipsing general apparel sales ever since. Euromonitor expects that pattern to continue: For 2013, sportswear sales grew an estimated 3.4%, while apparel inched up 0.9%. This year sportswear is projected to grow 3.3%; apparel 1.5%.

The growing sportswear sector has attracted new retailers, but more traditional apparel stores are also trying to get in on the action. In January, Swedish retailer H&M launched a new sportswear line focused on “functionality and comfort” that features a yoga tank top for $24.95 and sports tights for $29.95. Gap (GPS) said its yoga-wear Athleta brand had a “breakout year” in 2013, and the company plans to open 30 more stand-alone Athleta stores this year. Macy’s (M) is in the midst of opening Finish Line shops featuring athletic footwear, apparel, and accessories in more than 450 of its department stores.

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Lululemon (LULU), known for its performance yoga apparel and equipment, moved into more casual apparel last month, launching a line of sundresses, pants, and tank tops targeted toward women who “don’t have time for a wardrobe that keeps forcing you to change,” according to a recent ad. Six of the line’s 12 items sold out just hours after it launched.

The first time athletic wear doubled as streetwear was in the 1980s, when Americans took up jogging and mall walking, says Michael Londrigan, dean of academic affairs and merchandising expert at LIM, a college that teaches the business of fashion. But back then, baggy sweatsuits were all the rage. “What we’re seeing now is more fashion-forward, they don’t look like the sloppy fleeces from the ‘80s.” The move toward “athleisure” also marks a shift in which there’s less shame associated with body-conscious attire, he says. “If you have a good figure, athletic wear is going to show it off.”