AbstractTrading is the implementation of an investment decision. After a portfolio decision has been made by a portfolio manager, it must be implemented, and especially for handling large orders and navigating stressful markets, specific skills and responsibilities are needed that require the expertise of a professional trader. However, the efficiency with which orders are handled and turned into trades depends, not just on traders’ abilities, but also on a market’s liquidity, on the design of the marketplace where shares are traded, and on the regulatory environment. In this chapter, we cover trading costs, liquidity, volatility, price discovery, market structure, and market structure regulation.