Retailers that sell high-end goods don’t have the luxury of standing still. And they’re not, as several are making acquisitions, expanding internationally and introducing new digital technology, both online and in their stores.

While Neiman Marcus Group Ltd. LLC remains the clear leader in online luxury retailing, competitors are gaining ground. Notable among them is Yoox Net-A-Porter Group SpA., the global luxury e-retailer formed from the March 2015 acquisition of Net-A-Porter by Yoox, a company based in Italy that built its business selling luxury goods online and hosting e-retail sites for other brands. Yoox Net-A-Porter became the second luxury retailer to top $1 billion in annual web sales and its strong growth makes it a contender to overtake Neiman Marcus in a few years.

The other notable newcomers in the Top 10 (see table, page 40) are two fast-growing Chinese online retailers of luxury goods, Shangpin.com and Zbird.com, both of which doubled their web sales in 2015. Two of the three slowest-growing retailers in the Top 10 are flash-sale retailers—those that sell generally overstock or out-of-season fashion items at a discount in limited-time sales that last only as long as the merchandise does. Gilt Groupe Inc. and Rue La La were among the high-flying flash-sale sites during the recession of 2008-9 when many luxury brands and retailers had lots of excess inventory. But the economic recovery has put pressure on this business model.