There are two unprecedented sources of power on the Internet today: centralized platforms and decentralized crowds. They feed off each other, but also struggle against each other. Their interdependence defines the modern Internet, and it defies easy classification into regulatory silos. The meme-stock investors who drove GameStop stock to absurd heights coordinated on Reddit, and poured into the market on Robinhood. Influencers make their living by making content go viral on Instagram and TikTok. Disinformation operations target Facebook groups; violent mobs coordinate on Telegram. The essays in this collection explore the complex and interlinked dynamics of platforms and crowds. Scholars of sociology, technology, economics, and law discuss the nature of online crowds, their motivations and psychology, their influence on platforms, and platforms’ influence on them. The essays offer a primer on the essential social dynamics of online crowds, and a foundation for informed platform regulation that takes those dynamics into account. Contributors: Jessica L. Beyer - Finn Brunton - Gabriella Coleman - Evelyn Douek - Charles Duan - James Grimmelmann - Nikolas Guggenberger - Bing He - Srijan Kumar - Alice Marwick - Paul Ohm - Rebecca Tushnet