WINDERMERE, Fla. ( Stockpickr) -- Corporate insiders sell their own companies' stock for a variety of reasons. They might need the money for a big personal purchase such as a house or to fund a child's wedding. Sometimes they sell as part of a planned selling program that they've put in place for diversification purposes. Other times they sell because they think their stock is fully valued. Some even dump their own stock because they're worried about the state of the economy and think shares could go down significantly.But they only buy their own shares for one reason: because they think the stock will go up substantially. I like to search the market for companies that are experiencing heavy insider buying. Remember, corporate insiders know the most about their own companies. They're in the trenches every day and know how their business is doing. They know if big contracts are being signed. They know if demand is picking up for their products and services. Big insider buying is a positive sign worthy of your attention. It could mean that stock is severely undervalued and ready to skyrocket. Related: Stocks With Rising Analyst Expectations Recently we've had a number of companies whose corporate insiders were buying large amounts of stock. These insiders might be buying because they like the future prospects for their company. They could be loading up because the recent market volatility has created an opportunity for them to get in on the cheap. Some could be making a long-term bet by buying up their shares right now. In all of these cases, they're buying because they think the stock will go up. And if they're dropping millions of dollars into it, they most likely think the stock will go up significantly from where they got in. Here's a look at a number of companies whose insiders have been loading up on their own stock recently, according to SEC filings.