Wal-Mart Stores is an irresistible retail force that has yet to meet any immovable objects. Bigger than Europe's Carrefour, Metro AG, and Tesco combined, it's the world's number one retailer with some 2.2 million employees. This also makes it the largest employer in the world. In the US, Wal-Mart operates more than 4,000 stores, including 3,000-plus Supercenters that sell groceries and general merchandise, 620 Sam's Club warehouses, and a growing number of smaller format stores.