Platforms—a dozen massive ones, and perhaps 500 smaller ones—play an increasingly critical role in the livelihoods of hundreds of millions of people around the world. In almost every economic sector, platforms have introduced new “multi-sided” markets, matching buyers and sellers in attention, goods, services, and labor at massive scales. Websites and electronic supply chains are replacing bazaars and storefronts. Gig work is augmenting salaried work. Algorithms and finely-tuned digital user experiences are supercharging traditional buyer/seller relationships.