"Two of the main functions of financial markets are to discover equilibrium prices, thus providing useful information to the economy, and to enable investors to realize mutually beneficial trades. For markets to serve these goals, it is necessary that i) investors be matched efficiently and trade at prices accurately reflecting relevant information, and that, ii) transactions be executed without counterparty or payment failure. These conditions can fail due to informational frictions (limited rationality, search costs or adverse selection) or opportunistic behaviour.I will study the following issues: How can different trading and post-trading mechanisms mitigate or worsen these problems? Are financial market equilibria information constrained optima? Is there a role for public policies? Is algorithmic trading economically and socially useful? Can investors and institutions’ opportunistic behaviour generate counterparty risk? How do informational frictions affect market liquidity? What are the welfare properties of such trading institutions as electronic order books, high-frequency algorithmic trading, margin deposits, and central clearing counterparties?To study these issues, I will rely on theoretical models, and empirical analyses of financial markets data."