There's a lot of confused comments on this thread. "Front running" in the strictest sense means illegal trading that involves taking advantage of trades that you know will happen and you have a responsibility not to exploit. "Front running" is also used informally to mean legally trading prior to trades that you anticipate happening. Studying the rules of an index and buy a stock just before its added to the index and index funds are required to buy it is "front running" in the second sense.