Traditional economic models postulate that markets equate aggregate supply and demand at a single equilibrium price: All market participants can trade at this price instantaneously, and their counterparty is both unspecified and irrelevant. Many modern financial markets don't square well with these models. They are fragmented or "over-the-counter" in nature, with certain participants playing an outsized role in facilitating trades, which occur at different prices and take different amounts of time to execute. The purpose of this conference is to gain a better understanding of the inner workings of these markets. The papers that will be included in a special issue of the Review of Economic Dynamics will draw on a variety of methodological approaches to explore the structure of over-the-counter asset markets, the patterns of trade that emerge, and the implications for liquidity, execution speed, prices, and resilience to shocks. By invitation only.