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Front running is the illegal practice of a stockbroker executing orders on a security for its own account while taking advantage of advance knowledge of pending orders from its customers. When orders previously submitted by its customers will predictably affect the price of the security, purchasing first for its own account gives the broker an unfair advantage, since it can expect to close out its position at a profit based on the new price level. The front running broker either buys for his own account (before filling customer buy orders that drive up the price), or sells (where the broker sells for its own account, before filling customer sell orders that drive down the price).
Allegations of front running occasionally arise in stock and commodity exchanges, in scandals concerning floor brokers and exchange specialists.
For example, suppose a broker receives an order from a customer to buy a large block -- say, 400,000 shares -- of some stock, but before placing the order for the customer, the broker buys 20,000 shares of the same stock for his own account at $100 per share, then afterward places the customer's order for 400,000 shares, driving the price up to $102 per share and allowing the broker to immediately sell his shares for, say, $101.75, generating a significant profit of $35,000 in just a short time. This $35,000 is likely to be just a part of the additional cost to the customer's purchase caused by the broker's self-dealing.
This example uses unusually large numbers to get the point across. In practice, computer trading splits up large orders into many smaller ones, making front-running more difficult to detect. Moreover, the U.S. Securities and Exchange Commission's 2001 change to pricing stock in pennies, rather than fractions of no less than 1/8 of a dollar, facilitated front running by reducing the extra amount that must be offered to step in front of other orders.
By front-running, the broker has put his or her own financial interest above (or in front of) the customer's interest and is thus committing fraud. In the United States, he or she might also be breaking laws on market manipulation or insider trading.
Other uses of the term 
Front-running may also occur in the context of insider trading, as when those close to the CEO of a firm act through short sales ahead of the announcement of a sale of stock by the CEO, which will in turn trigger a drop in the stock's price. Khan & Lu (2008: 1) define front running as "trading by some parties in advance of large trades by other parties, in anticipation of profiting from the price movement that follows the large trade". They find evidence consistent with front-running through short sales ahead of large stock sales by CEOs on the New York Stock Exchange.
While front-running is illegal when a broker uses private information about a client's pending order, in principle it is not illegal if it is based on public information. In his book Trading & Exchanges, Larry Harris outlines several other related types of trading. Though all these types of trading may not be strictly illegal, he terms them "parasitic".
A third-party trader may find out the content of another broker's order and buy or sell in front of it in the same way that a self-dealing broker might. The third-party trader might find out about the trade directly from the broker or an employee of the brokerage firm in return for splitting the profits, in which case the front-running would be illegal. The trader might, however, only find out about the order by reading the broker's habits or tics, much in the same way that poker players can guess other players' cards. For very large market orders, simply exposing the order to the market, may cause traders to front-run as they seek to close out positions that may soon become unprofitable.
Large limit orders can be "front-run" by "order matching" or "penny jumping". For example if a buy limit order for 100,000 shares for $1.00 is announced to the market, many traders may seek to buy for $1.01. If the market price increases after their purchases, they will get the full amount of the price increase. However, if the market price decreases, they will likely be able to sell to the limit order trader, for only a one cent loss. This type of trading is probably not illegal, and in any case, a law against it would be very difficult to enforce. Harris still considers it "parasitic".
Other types of traders who use generally similar strategies are labelled "order anticipators" by Harris. These include "sentiment-oriented technical traders," traders who buy during an asset bubble even though they know the asset is overpriced, and squeezers who drive up prices by threatening to corner the market. Squeezers would likely be guilty of market manipulation, but the other two types of order anticipators would not be violating any US law.
"Front running" is sometimes used informally for a broker's tactics related to trading on proprietary information before its clients have been given the information.
For example, analysts and brokers who buy shares in a company just before the brokerage firm is about to recommend the stock as a strong buy, are practicing this type of "front running". Brokers have been convicted of securities laws violations in the United States for such behavior. In 1985, a writer for the Wall Street Journal, R. Foster Winans, tipped off brokers about the content of his column Heard on the Street, which based upon publicly available information would be written in such a way as to give either good or bad news about various stocks. The tipped off brokers traded on the information. Winans and the brokers were prosecuted by then-prosecutor Rudolph Giuliani, tried and convicted of securities fraud. Their convictions were upheld by the United States Supreme Court in 1986.
See also 
- Larry Harris, Trading & Exchanges, Oxford Press, Oxford, 2003. Chapter 11 "Order Anticipators."
- Khan, Mozaffar and Lu, Hai, Do Short Sellers Front-Run Insider Sales?(August 1, 2008). MIT Sloan Research Paper No. 4706-08. Available at SSRN: http://ssrn.com/abstract=1140694
- * Carpenter v. United States, Supreme Court decision
- * High-Frequency Trading Prospers at Expense of Everyone, Bloomberg editorial