Amazon and Walmart, with more than a trillion dollars in annual revenues combined, are the two largest companies in the world. In the coming years, Amazon will probably dethrone Walmart as the world's largest company, a position that Walmart has occupied for more than two decades. By examining these two companies and their business models in depth, Nirmalya Kumar elucidates the more general phenomenon of incumbents competing with disruptors (e.g., Volkswagen vs Tesla, Marriott vs Airbnb) as well as the move to omnichannel retail, where physical stores must coexist with online retailers.