In trading terms, the difference between the prevailing price or value when a buy or sell decision is made with regard to a security and the final execution price or value after taking into consideration all commissions, fees and taxes. As such, implementation shortfall is the sum of execution costs and the opportunity cost incurred in case of adverse market movement between the time of the trading decision and order execution.

In order to maximize the potential for profit, investors aim to keep implementation shortfall as low as possible. Investors have been helped in this endeavor over the past two decades by developments such as discount brokerages, online trading and access to real-time quotes and information.