According to the U.S. SEC, corporate insiders are a company's officers, directors and any beneficial owners of more than ten percent of a class of the company's equity securities. Trades made by these types of insiders in the company's own stock, based on material non-public information, are considered to be fraudulent since the insiders are violating the trust or the fiduciary duty that they owe to the shareholders. The corporate insider, simply by accepting employment, has made a contract with the shareholders to put the shareholders' interests before their own, in matters related to the corporation. When the insider buys or sells based upon company owned information, he is violating his contract with the shareholders.

For example, illegal insider trading would occur if the chief executive officer of Company A learned (prior to a public announcement) that Company A will be taken over, and bought shares in Company A knowing that the share price would likely rise.

Liability for insider trading violations cannot be avoided by passing on the information in an "I scratch your back, you scratch mine" or quid pro quo arrangement, as long as the person receiving the information knew or should have known that the information was company property. For example, if Company A's CEO did not trade on the undisclosed takeover news, but instead passed the information on to his brother-in-law who traded on it, illegal insider trading would still have occurred.

A newer view of insider trading, the "misappropriation theory" is now part of US law. It states that anyone who misappropriates (steals) information from their employer and trades on that information in any stock (not just the employer's stock) is guilty of insider trading. For example, if a journalist who worked for Company B learned about the takeover of Company A while performing his work duties, and bought stock in Company A, illegal insider trading might still have occurred. Even though the journalist did not violate a fiduciary duty to Company A's shareholders, he might have violated a fiduciary duty to Company B's shareholders (assuming the newspaper had a policy of not allowing reporters to trade on stories they were covering).

Proving that someone has been responsible for a trade can be difficult, because traders may try to hide behind nominees, offshore companies, and other proxies. Nevertheless, the U.S. Securities and Exchange Commission prosecutes over 50 cases each year, with many being settled administratively out of court. The SEC and several stock exchanges actively monitor trading, looking for suspicious activity.

Not all trading on information is illegal inside trading, however. For example, while dining at a restaurant, you hear the CEO of Company A at the next table telling the CFO that the company will be taken over, and then you buy the stock, you wouldn't be guilty of insider trading unless there was some closer connection between you, the company, or the company officers.

Since insiders are required to report their trades, others often track these traders, and there is a school of investing which follows the lead of insiders. This is of course subject to the risk that an insider is making a buy specifically to increase investor confidence, or making a sell for reasons unrelated to the health of the company (e.g. a desire to diversify or buy a house).

As of December 2005 companies are required to announce times to their employees as to when they can safely trade without being accused of trading on inside information.