Chapter 1: Bayesian Persuasion in the Digital Age In digital platforms, agents have vast access to information, but the quality of it is unclear. This paper studies the impact of uncertainty about the quality of information as a Bayesian persuasion game with multiple senders. The model innovates by assuming the receiver samples only a small subset of the information available and whose quality uncertainty is endogenous. The receiver knows the signals chosen by each sender but randomly observes the realization of only one such signal. In equilibrium, senders can pool their signals, so the receiver is uncertain about the informativeness of the message received. Since pooling is prevalent, each sender chooses a signal to affect the average correlation between messages and states of the world. Thus, the strategies of other senders constrain each sender's ability to design information and may incentivize a sender to provide more or less information compared to the single-sender information design benchmark. The model suggests policies to improve the quality of the information on platforms such as social media. Chapter 2: Dynamic Trading in Decentralized Markets (with Marzena Rostek) Most financial assets are traded in multiple venues. We study a model of imperfectly competitive trading where agents have multiple opportunities to trade risky assets. We consider decentralized markets: the market comprises coexisting exchanges, each defined by a subset of traders and assets. We characterize the equilibrium dynamics of prices, trades, and price impact. Markets with the same prices, allocations, and price impact may differ in their dynamic efficiency properties depending on traders' participation in different exchanges. We provide necessary and sufficient conditions - based on trader participation alone - for markets to be dynamically efficient as trading becomes frequent. Decentralized markets for a single asset (or, more generally, standardized assets) are always dynamically efficient. For assets traded at low frequencies, even markets that are not dynamically efficient can give rise to higher total welfare than the centralized market. Increasing trading frequency can lower welfare due to the interaction of price impact and market incompleteness (i.e., limited participation in the exchanges). Chapter 3: Games among Groups (with Marzena Rostek and Ji Hee Yoon) In many social interactions, externalities and peer effects apply to groups rather than all agents. We model noncooperative games among a finite number of players who interact through any number of groups (a hypergraph). Groups endogenously change how the players value the interaction and contribute to it. Interaction through groups can realize a surplus that cannot be attained by a single group among the same players. Private clubs (interactions among some agents) can increase welfare if they are exclusive and not too prevalent. Different group interactions enhance equilibrium surplus through externality vs. risk-sharing motives.