This paper reviews and analyzes the legal and economic aspects of the duty of best execution. Although a well-established principle of securities trading, we show that the dual problems of definition and enforcement make best execution both unwieldy and unworkable as a mandated legal duty. We examine the impact of several market practices on best execution, in particular payment for order flow, preferencing and internalization practices, and price improvement and order execution protocols. We suggest three possible directions for the future rule and interpretation of the duty of best execution.Journal of Economic LiteratureClassification Numbers: G10, G18, K22, K23.

Using the model structure of Easley and O'Hara (Journal of Finance, 47, 577–604), we demonstrate how the parameters of the market-maker's beliefs can be estimated from trade data. We show how to extract information from both trade and no-trade intervals, and how intraday and interday data provide information. We derive and evaluate tests of model specification and estimate the information content of differential trade sizes. Our work provides a framework for testing extant microstructure models, shows how to extract the information contained in the trading process, and demonstrates the empirical importance of asymmetric information models for asset prices.

Using the model structure of Easley and O'Hara (Journal of Finance, 47, 577–604), we demonstrate how the parameters of the market-maker's beliefs can be estimated from trade data. We show how to extract information from both trade and no-trade intervals, and how intraday and interday data provide information. We derive and evaluate tests of model specification and estimate the information content of differential trade sizes. Our work provides a framework for testing extant microstructure models, shows how to extract the information contained in the trading process, and demonstrates the empirical importance of symmetric information models for asset prices.