Historically, Walmart's edge over physical retail competitors has been the efficiency of its logistics network. The retailer currently has 7,500 drivers, and a fleet of 6,500 trucks that transport goods to and from 150 distribution centers that serve its nearly 5,400 stores (including Sam's Club locations) across the U.S. The challenge for Walmart is to evolve its logistics model to an e-commerce age when consumers are less likely to wait for an out-of-stock item and buy it from a competitor online.