Insiders - the officers, board members and major shareholders of America's corporations - are required by law to almost immediately report to the Securities and Exchange Commission (SEC) any time they buy or sell the shares of their own companies. As such, insider transactions are tracked by a number of organizations and Wall Street analysts as a gauge of current market sentiment and future prospects for stock prices.
The theory underlying this practice is simple. As the people with the most intimate knowledge of what corporations are actually doing to grow their businesses, as well as the results those strategies are producing, insiders are in the best position to judge whether the fortunes of their companies are looking bright - or dismal. When they like what they see, they buy their company's shares - and when they don't, they sell.