If you saw a group of millennials out on a Saturday in the mid-2010s, they were most likely wearing leggings—the uniform of that era. And there’s a good chance they were Lululemon’s Align leggings.

Ten years ago today, Lululemon’s designers developed a new material called Nulu that was buttery soft, thin, and stretchy. It put them into a $98 pair of leggings called Align. The fabric proved so irresistible that women started wearing the pants right out of the yoga studio and into the rest of their lives. On the newly launched Instagram app, you would see twenty- and thirtysomethings wearing the pricey leggings out to brunch, or for school pickups, or on long flights. Some women even wore them to the office with a crisp button-down oxford shirt.

If the Align legging didn’t kick-start athleisure—blending activewear with everyday outfits—then it certainly accelerated the trend. And it helped propel Lululemon from a scrappy yoga startup into a global activewear giant. Over the past decade, Lululemon has generated more than $1 billion on its Align franchise. And it has spawned so many copycats from other brands that two years ago it launched an entire “dupe swap” marketing campaign, where it invited people to trade in their dupes to get the real thing.

Chip Wilson, a yoga obsessive, launched Lululemon in Vancouver, British Columbia, in 1998. For its first decade in business, the company remained small and focused on designing high-quality clothing that was perfect for yoga, which was taking off throughout North America. But by 2010, the company’s growth accelerated as it went public and began expanding its network of stores. In 2012, Lululemon hit $1 billion in annual revenue. After the Align leggings launched, the company scaled quickly, growing from $1.8 billion to $10.6 billion over the past decade.

Today, Lululemon is celebrating the anniversary of the Align pants by launching a range of new products, including a dress and a new version of the pants that offer a seamless construction in the front, developed with customer feedback. But the company is also thinking ahead about how it must evolve beyond this comfortable legging into entirely new categories of clothing.

I sat down with Antonia Iamartino, senior director of........

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