Zipping around the floor of a Crate & Barrel warehouse in Tracy, Calif., a cadre of 35 orange robots helps solitary human employees do the work of six people. The squat machines carry shelves with the company’s 8,000 different products to people, who pick just the items they need to prepare online orders for shipment—all without walking around the sprawling building. While Crate & Barrel sees orders quadruple during the holiday weeks, this warehouse gets by with just double the employees, about 90 humans, thanks to the robots.

Meanwhile, just outside of Phoenix, Inc. takes a more human-powered approach. There, employees walk 18 to 20 miles a day down aisles lined with shelves, filling library-style carts with the latest orders and carrying items back to packing stations. For the holiday season, this Phoenix facility—one of about 20 that Amazon has across the U.S.—quadruples its staff to about 1,200 to handle the holiday rush, with teams working 24 hours a day.

Which approach is better is a matter of debate in the 15-year-old e-commerce industry. Amazon has avoided robots at its fulfillment centers, aside from systems that came with recent acquisitions such as Yet Kiva Systems Inc., the company that makes the robots used by Crate & Barrel, now has systems in place at several top online retailers in the U.S., including Gap Inc., Staples Inc. and Gilt Groupe Inc.

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